diff --git a/Libraries/LibJS/Runtime/Date.cpp b/Libraries/LibJS/Runtime/Date.cpp
index ab38337274..322fa0c0f8 100644
--- a/Libraries/LibJS/Runtime/Date.cpp
+++ b/Libraries/LibJS/Runtime/Date.cpp
@@ -7,8 +7,8 @@
 
 #include <AK/NeverDestroyed.h>
 #include <AK/NumericLimits.h>
-#include <AK/StringBuilder.h>
 #include <AK/Time.h>
+#include <AK/Utf16StringBuilder.h>
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/Date.h>
 #include <LibJS/Runtime/GlobalObject.h>
@@ -36,30 +36,30 @@ Date::Date(double date_value, Object& prototype)
 
 Date::~Date() = default;
 
-ErrorOr<String> Date::iso_date_string() const
+ErrorOr<Utf16String> Date::iso_date_string() const
 {
     int year = year_from_time(m_date_value);
 
-    StringBuilder builder;
+    Utf16StringBuilder builder;
     if (year < 0)
         builder.appendff("-{:06}", -year);
     else if (year > 9999)
         builder.appendff("+{:06}", year);
     else
         builder.appendff("{:04}", year);
-    builder.append('-');
+    builder.append_ascii('-');
     builder.appendff("{:02}", month_from_time(m_date_value) + 1);
-    builder.append('-');
+    builder.append_ascii('-');
     builder.appendff("{:02}", date_from_time(m_date_value));
-    builder.append('T');
+    builder.append_ascii('T');
     builder.appendff("{:02}", hour_from_time(m_date_value));
-    builder.append(':');
+    builder.append_ascii(':');
     builder.appendff("{:02}", min_from_time(m_date_value));
-    builder.append(':');
+    builder.append_ascii(':');
     builder.appendff("{:02}", sec_from_time(m_date_value));
-    builder.append('.');
+    builder.append_ascii('.');
     builder.appendff("{:03}", ms_from_time(m_date_value));
-    builder.append('Z');
+    builder.append_ascii('Z');
 
     return builder.to_string();
 }
diff --git a/Libraries/LibJS/Runtime/Date.h b/Libraries/LibJS/Runtime/Date.h
index ba87d95d00..ceec41c261 100644
--- a/Libraries/LibJS/Runtime/Date.h
+++ b/Libraries/LibJS/Runtime/Date.h
@@ -27,7 +27,7 @@ public:
     double date_value() const { return m_date_value; }
     void set_date_value(double value) { m_date_value = value; }
 
-    ErrorOr<String> iso_date_string() const;
+    ErrorOr<Utf16String> iso_date_string() const;
 
 private:
     Date(double date_value, Object& prototype);
diff --git a/Libraries/LibJS/Runtime/JSONObject.cpp b/Libraries/LibJS/Runtime/JSONObject.cpp
index 455642b251..2b302e719c 100644
--- a/Libraries/LibJS/Runtime/JSONObject.cpp
+++ b/Libraries/LibJS/Runtime/JSONObject.cpp
@@ -6,7 +6,6 @@
 
 #include <AK/Function.h>
 #include <AK/GenericLexer.h>
-#include <AK/StringBuilder.h>
 #include <AK/StringConversions.h>
 #include <AK/TypeCasts.h>
 #include <AK/Utf16StringBuilder.h>
@@ -52,7 +51,7 @@ void JSONObject::initialize(Realm& realm)
 }
 
 // 25.5.2 JSON.stringify ( value [ , replacer [ , space ] ] ), https://tc39.es/ecma262/#sec-json.stringify
-ThrowCompletionOr<Optional<String>> JSONObject::stringify_impl(VM& vm, Value value, Value replacer, Value space)
+ThrowCompletionOr<Optional<Utf16String>> JSONObject::stringify_impl(VM& vm, Value value, Value replacer, Value space)
 {
     auto& realm = *vm.current_realm();
 
@@ -99,15 +98,15 @@ ThrowCompletionOr<Optional<String>> JSONObject::stringify_impl(VM& vm, Value val
     if (space.is_number()) {
         auto space_mv = MUST(space.to_integer_or_infinity(vm));
         space_mv = min(10, space_mv);
-        state.gap = space_mv < 1 ? String {} : MUST(String::repeated(' ', space_mv));
+        state.gap = space_mv < 1 ? Utf16String {} : Utf16String::repeated(' ', space_mv);
     } else if (space.is_string()) {
-        auto string = space.as_string().utf8_string();
-        if (string.bytes().size() <= 10)
+        auto string = space.as_string().utf16_string();
+        if (string.length_in_code_units() <= 10)
             state.gap = string;
         else
-            state.gap = MUST(string.substring_from_byte_offset(0, 10));
+            state.gap = Utf16String::from_utf16(string.utf16_view().substring_view(0, 10));
     } else {
-        state.gap = String {};
+        state.gap = Utf16String {};
     }
 
     auto wrapper = Object::create(realm, realm.intrinsics().object_prototype());
@@ -115,9 +114,9 @@ ThrowCompletionOr<Optional<String>> JSONObject::stringify_impl(VM& vm, Value val
 
     bool wrote_value = TRY(serialize_json_property(vm, state, Utf16String {}, wrapper));
     if (!wrote_value)
-        return Optional<String> {};
+        return Optional<Utf16String> {};
 
-    return state.builder.to_string_without_validation();
+    return state.builder.to_string();
 }
 
 // 25.5.2 JSON.stringify ( value [ , replacer [ , space ] ] ), https://tc39.es/ecma262/#sec-json.stringify
@@ -172,7 +171,7 @@ ThrowCompletionOr<bool> JSONObject::serialize_json_property(VM& vm, StringifySta
         // a. If value has an [[IsRawJSON]] internal slot, then
         if (is<RawJSONObject>(value_object)) {
             // i. Return ! Get(value, "rawJSON").
-            builder.append(MUST(value_object.get(vm.names.rawJSON)).as_string().utf8_string());
+            builder.append(MUST(value_object.get(vm.names.rawJSON)).as_string().utf16_string_view());
             return true;
         }
         // b. If value has a [[NumberData]] internal slot, then
@@ -199,14 +198,17 @@ ThrowCompletionOr<bool> JSONObject::serialize_json_property(VM& vm, StringifySta
 
     // 5. If value is null, return "null".
     if (value.is_null()) {
-        builder.append("null"sv);
+        builder.append_ascii("null"sv);
         return true;
     }
 
     // 6. If value is true, return "true".
     // 7. If value is false, return "false".
     if (value.is_boolean()) {
-        builder.append(value.as_bool() ? "true"sv : "false"sv);
+        if (value.as_bool())
+            builder.append_ascii("true"sv);
+        else
+            builder.append_ascii("false"sv);
         return true;
     }
 
@@ -220,12 +222,12 @@ ThrowCompletionOr<bool> JSONObject::serialize_json_property(VM& vm, StringifySta
     if (value.is_number()) {
         // a. If value is finite, return ! ToString(value).
         if (value.is_finite_number()) {
-            number_to_string(builder, value.as_double());
+            builder.append(number_to_utf16_string(value.as_double()));
             return true;
         }
 
         // b. Return "null".
-        builder.append("null"sv);
+        builder.append_ascii("null"sv);
         return true;
     }
 
@@ -253,10 +255,10 @@ ThrowCompletionOr<bool> JSONObject::serialize_json_property(VM& vm, StringifySta
     return false;
 }
 
-static void write_indent(StringBuilder& builder, StringView gap, size_t depth)
+static void write_indent(Utf16StringBuilder& builder, Utf16String const& gap, size_t depth)
 {
     for (size_t i = 0; i < depth; ++i)
-        builder.append(gap);
+        builder.append(gap.utf16_view());
 }
 
 // 25.5.2.4 SerializeJSONObject ( state, value ), https://tc39.es/ecma262/#sec-serializejsonobject
@@ -272,8 +274,8 @@ ThrowCompletionOr<void> JSONObject::serialize_json_object(VM& vm, StringifyState
     ++state.indent_depth;
 
     auto& builder = state.builder;
-    builder.append('{');
-    size_t position_after_open_brace = builder.length();
+    builder.append_ascii('{');
+    size_t position_after_open_brace = builder.length_in_code_units();
     bool first = true;
 
     auto process_property = [&](PropertyKey const& key) -> ThrowCompletionOr<void> {
@@ -281,25 +283,25 @@ ThrowCompletionOr<void> JSONObject::serialize_json_object(VM& vm, StringifyState
             return {};
 
         // Mark position before writing anything for this property
-        size_t mark = builder.length();
+        size_t mark = builder.length_in_code_units();
 
         // Write separator (comma and possibly newline/indent)
         if (!first) {
-            builder.append(',');
+            builder.append_ascii(',');
             if (!state.gap.is_empty()) {
-                builder.append('\n');
+                builder.append_ascii('\n');
                 write_indent(builder, state.gap, state.indent_depth);
             }
         } else if (!state.gap.is_empty()) {
-            builder.append('\n');
+            builder.append_ascii('\n');
             write_indent(builder, state.gap, state.indent_depth);
         }
 
         // Write key and colon
         quote_json_string(builder, key.to_string());
-        builder.append(':');
+        builder.append_ascii(':');
         if (!state.gap.is_empty())
-            builder.append(' ');
+            builder.append_ascii(' ');
 
         // Serialize value
         bool wrote_value = TRY(serialize_json_property(vm, state, key, &object));
@@ -308,7 +310,7 @@ ThrowCompletionOr<void> JSONObject::serialize_json_object(VM& vm, StringifyState
             first = false;
         } else {
             // Rollback - value was undefined, remove everything we wrote for this property
-            builder.trim(builder.length() - mark);
+            builder.trim(builder.length_in_code_units() - mark);
         }
         return {};
     };
@@ -325,11 +327,11 @@ ThrowCompletionOr<void> JSONObject::serialize_json_object(VM& vm, StringifyState
 
     // Close the object
     --state.indent_depth;
-    if (builder.length() > position_after_open_brace && !state.gap.is_empty()) {
-        builder.append('\n');
+    if (builder.length_in_code_units() > position_after_open_brace && !state.gap.is_empty()) {
+        builder.append_ascii('\n');
         write_indent(builder, state.gap, state.indent_depth);
     }
-    builder.append('}');
+    builder.append_ascii('}');
 
     state.seen_objects.remove(&object);
     return {};
@@ -350,44 +352,44 @@ ThrowCompletionOr<void> JSONObject::serialize_json_array(VM& vm, StringifyState&
     auto& builder = state.builder;
     auto length = TRY(length_of_array_like(vm, object));
 
-    builder.append('[');
+    builder.append_ascii('[');
 
     for (size_t i = 0; i < length; ++i) {
         // Write separator
         if (i > 0) {
-            builder.append(',');
+            builder.append_ascii(',');
             if (!state.gap.is_empty()) {
-                builder.append('\n');
+                builder.append_ascii('\n');
                 write_indent(builder, state.gap, state.indent_depth);
             }
         } else if (!state.gap.is_empty()) {
-            builder.append('\n');
+            builder.append_ascii('\n');
             write_indent(builder, state.gap, state.indent_depth);
         }
 
         // Serialize value (undefined becomes null for arrays)
         bool wrote_value = TRY(serialize_json_property(vm, state, i, &object));
         if (!wrote_value)
-            builder.append("null"sv);
+            builder.append_ascii("null"sv);
     }
 
     // Close the array
     --state.indent_depth;
     if (length > 0 && !state.gap.is_empty()) {
-        builder.append('\n');
+        builder.append_ascii('\n');
         write_indent(builder, state.gap, state.indent_depth);
     }
-    builder.append(']');
+    builder.append_ascii(']');
 
     state.seen_objects.remove(&object);
     return {};
 }
 
 // 25.5.2.2 QuoteJSONString ( value ), https://tc39.es/ecma262/#sec-quotejsonstring
-void JSONObject::quote_json_string(StringBuilder& builder, Utf16View const& string)
+void JSONObject::quote_json_string(Utf16StringBuilder& builder, Utf16View const& string)
 {
     // 1. Let product be the String value consisting solely of the code unit 0x0022 (QUOTATION MARK).
-    builder.append('"');
+    builder.append_ascii('"');
 
     // 2. For each code point C of StringToCodePoints(value), do
     for (auto code_point : string) {
@@ -395,25 +397,25 @@ void JSONObject::quote_json_string(StringBuilder& builder, Utf16View const& stri
         // i. Set product to the string-concatenation of product and the escape sequence for C as specified in the "Escape Sequence" column of the corresponding row.
         switch (code_point) {
         case '\b':
-            builder.append("\\b"sv);
+            builder.append_ascii("\\b"sv);
             break;
         case '\t':
-            builder.append("\\t"sv);
+            builder.append_ascii("\\t"sv);
             break;
         case '\n':
-            builder.append("\\n"sv);
+            builder.append_ascii("\\n"sv);
             break;
         case '\f':
-            builder.append("\\f"sv);
+            builder.append_ascii("\\f"sv);
             break;
         case '\r':
-            builder.append("\\r"sv);
+            builder.append_ascii("\\r"sv);
             break;
         case '"':
-            builder.append("\\\""sv);
+            builder.append_ascii("\\\""sv);
             break;
         case '\\':
-            builder.append("\\\\"sv);
+            builder.append_ascii("\\\\"sv);
             break;
         default:
             // b. Else if C has a numeric value less than 0x0020 (SPACE), or if C has the same numeric value as a leading surrogate or trailing surrogate, then
@@ -431,7 +433,7 @@ void JSONObject::quote_json_string(StringBuilder& builder, Utf16View const& stri
     }
 
     // 3. Set product to the string-concatenation of product and the code unit 0x0022 (QUOTATION MARK).
-    builder.append('"');
+    builder.append_ascii('"');
 }
 
 // 25.5.1 JSON.parse ( text [ , reviver ] ), https://tc39.es/ecma262/#sec-json.parse
diff --git a/Libraries/LibJS/Runtime/JSONObject.h b/Libraries/LibJS/Runtime/JSONObject.h
index c2f037690c..15a17a8669 100644
--- a/Libraries/LibJS/Runtime/JSONObject.h
+++ b/Libraries/LibJS/Runtime/JSONObject.h
@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/Utf16StringBuilder.h>
 #include <LibJS/Export.h>
 #include <LibJS/Runtime/Object.h>
 
@@ -38,7 +39,7 @@ public:
 
     // The base implementation of stringify is exposed because it is used by
     // test-js to communicate between the JS tests and the C++ test runner.
-    static ThrowCompletionOr<Optional<String>> stringify_impl(VM&, Value value, Value replacer, Value space);
+    static ThrowCompletionOr<Optional<Utf16String>> stringify_impl(VM&, Value value, Value replacer, Value space);
 
     static ThrowCompletionOr<Value> parse_json(VM&, StringView text, JSONParseRecord* root_record = nullptr);
 
@@ -49,16 +50,16 @@ private:
         GC::Ptr<FunctionObject> replacer_function;
         HashTable<GC::Ptr<Object>> seen_objects;
         size_t indent_depth { 0 };
-        String gap;
+        Utf16String gap;
         Optional<Vector<Utf16String>> property_list;
-        StringBuilder builder;
+        Utf16StringBuilder builder;
     };
 
     // Stringify helpers
     static ThrowCompletionOr<bool> serialize_json_property(VM&, StringifyState&, PropertyKey const& key, Object* holder);
     static ThrowCompletionOr<void> serialize_json_object(VM&, StringifyState&, Object&);
     static ThrowCompletionOr<void> serialize_json_array(VM&, StringifyState&, Object&);
-    static void quote_json_string(StringBuilder&, Utf16View const&);
+    static void quote_json_string(Utf16StringBuilder&, Utf16View const&);
 
     // Parse helpers
     static ThrowCompletionOr<Value> internalize_json_property(VM&, Object* holder, PropertyKey const& name, FunctionObject& reviver, JSONParseRecord const* parse_record);
diff --git a/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp b/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp
index de27ecc8a9..51ee087f90 100644
--- a/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp
+++ b/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp
@@ -8,8 +8,8 @@
  */
 
 #include <AK/NeverDestroyed.h>
-#include <AK/StringBuilder.h>
 #include <AK/Utf16String.h>
+#include <AK/Utf16StringBuilder.h>
 #include <LibCrypto/BigFraction/BigFraction.h>
 #include <LibJS/Runtime/Date.h>
 #include <LibJS/Runtime/PropertyKey.h>
@@ -757,44 +757,44 @@ ThrowCompletionOr<bool> is_partial_temporal_object(VM& vm, Value value)
 }
 
 // 13.25 FormatFractionalSeconds ( subSecondNanoseconds, precision ), https://tc39.es/proposal-temporal/#sec-temporal-formatfractionalseconds
-String format_fractional_seconds(u64 sub_second_nanoseconds, Precision precision)
+Utf16String format_fractional_seconds(u64 sub_second_nanoseconds, Precision precision)
 {
-    String fraction_string;
+    Utf16String fraction_string;
 
     // 1. If precision is auto, then
     if (precision.has<Auto>()) {
         // a. If subSecondNanoseconds = 0, return the empty String.
         if (sub_second_nanoseconds == 0)
-            return String {};
+            return Utf16String {};
 
         // b. Let fractionString be ToZeroPaddedDecimalString(subSecondNanoseconds, 9).
-        fraction_string = MUST(String::formatted("{:09}", sub_second_nanoseconds));
+        fraction_string = Utf16String::formatted("{:09}", sub_second_nanoseconds);
 
         // c. Set fractionString to the longest prefix of fractionString ending with a code unit other than 0x0030 (DIGIT ZERO).
-        fraction_string = MUST(fraction_string.trim("0"sv, TrimMode::Right));
+        fraction_string = Utf16String::from_utf16(fraction_string.utf16_view().trim("0"sv, TrimMode::Right));
     }
     // 2. Else,
     else {
         // a. If precision = 0, return the empty String.
         if (precision.get<u8>() == 0)
-            return String {};
+            return Utf16String {};
 
         // b. Let fractionString be ToZeroPaddedDecimalString(subSecondNanoseconds, 9).
-        fraction_string = MUST(String::formatted("{:09}", sub_second_nanoseconds));
+        fraction_string = Utf16String::formatted("{:09}", sub_second_nanoseconds);
 
         // c. Set fractionString to the substring of fractionString from 0 to precision.
-        fraction_string = MUST(fraction_string.substring_from_byte_offset(0, precision.get<u8>()));
+        fraction_string = Utf16String::from_utf16(fraction_string.utf16_view().substring_view(0, precision.get<u8>()));
     }
 
     // 3. Return the string-concatenation of the code unit 0x002E (FULL STOP) and fractionString.
-    StringBuilder builder;
-    builder.append('.');
-    builder.append(fraction_string);
-    return MUST(builder.to_string());
+    Utf16StringBuilder builder;
+    builder.append_ascii('.');
+    builder.append(fraction_string.utf16_view());
+    return builder.to_string();
 }
 
 // 13.26 FormatTimeString ( hour, minute, second, subSecondNanoseconds, precision [ , style ] ), https://tc39.es/proposal-temporal/#sec-temporal-formattimestring
-String format_time_string(u8 hour, u8 minute, u8 second, u64 sub_second_nanoseconds, SecondsStringPrecision::Precision precision, Optional<TimeStyle> style)
+Utf16String format_time_string(u8 hour, u8 minute, u8 second, u64 sub_second_nanoseconds, SecondsStringPrecision::Precision precision, Optional<TimeStyle> style)
 {
     // 1. If style is present and style is UNSEPARATED, let separator be the empty String; else, let separator be ":".
     auto separator = style == TimeStyle::Unseparated ? ""sv : ":"sv;
@@ -804,14 +804,14 @@ String format_time_string(u8 hour, u8 minute, u8 second, u64 sub_second_nanoseco
 
     // 4. If precision is minute, return the string-concatenation of hh, separator, and mm.
     if (precision.has<SecondsStringPrecision::Minute>())
-        return MUST(String::formatted("{:02}{}{:02}", hour, separator, minute));
+        return Utf16String::formatted("{:02}{}{:02}", hour, separator, minute);
 
     // 5. Let ss be ToZeroPaddedDecimalString(second, 2).
     // 6. Let subSecondsPart be FormatFractionalSeconds(subSecondNanoseconds, precision).
     auto sub_seconds_part = format_fractional_seconds(sub_second_nanoseconds, precision.downcast<Auto, u8>());
 
     // 7. Return the string-concatenation of hh, separator, mm, separator, ss, and subSecondsPart.
-    return MUST(String::formatted("{:02}{}{:02}{}{:02}{}", hour, separator, minute, separator, second, sub_seconds_part));
+    return Utf16String::formatted("{:02}{}{:02}{}{:02}{}", hour, separator, minute, separator, second, sub_seconds_part);
 }
 
 // 13.27 GetUnsignedRoundingMode ( roundingMode, sign ), https://tc39.es/proposal-temporal/#sec-getunsignedroundingmode
diff --git a/Libraries/LibJS/Runtime/Temporal/AbstractOperations.h b/Libraries/LibJS/Runtime/Temporal/AbstractOperations.h
index 75b5f48f21..01a6892783 100644
--- a/Libraries/LibJS/Runtime/Temporal/AbstractOperations.h
+++ b/Libraries/LibJS/Runtime/Temporal/AbstractOperations.h
@@ -183,8 +183,8 @@ UnitCategory temporal_unit_category(Unit);
 RoundingIncrement maximum_temporal_duration_rounding_increment(Unit);
 Crypto::UnsignedBigInteger const& temporal_unit_length_in_nanoseconds(Unit);
 ThrowCompletionOr<bool> is_partial_temporal_object(VM&, Value);
-String format_fractional_seconds(u64, Precision);
-String format_time_string(u8 hour, u8 minute, u8 second, u64 sub_second_nanoseconds, SecondsStringPrecision::Precision, Optional<TimeStyle> = {});
+Utf16String format_fractional_seconds(u64, Precision);
+Utf16String format_time_string(u8 hour, u8 minute, u8 second, u64 sub_second_nanoseconds, SecondsStringPrecision::Precision, Optional<TimeStyle> = {});
 UnsignedRoundingMode get_unsigned_rounding_mode(RoundingMode, Sign);
 double apply_unsigned_rounding_mode(double, double r1, double r2, UnsignedRoundingMode);
 Crypto::SignedBigInteger apply_unsigned_rounding_mode(Crypto::SignedDivisionResult const&, Crypto::SignedBigInteger r1, Crypto::SignedBigInteger r2, UnsignedRoundingMode, Crypto::UnsignedBigInteger const& increment);
diff --git a/Libraries/LibJS/Runtime/Temporal/Duration.cpp b/Libraries/LibJS/Runtime/Temporal/Duration.cpp
index b6bf3a51e7..fcbee978af 100644
--- a/Libraries/LibJS/Runtime/Temporal/Duration.cpp
+++ b/Libraries/LibJS/Runtime/Temporal/Duration.cpp
@@ -10,6 +10,7 @@
 #include <AK/GenericShorthands.h>
 #include <AK/Math.h>
 #include <AK/NumericLimits.h>
+#include <AK/Utf16StringBuilder.h>
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/Date.h>
 #include <LibJS/Runtime/Intrinsics.h>
@@ -1474,13 +1475,13 @@ ThrowCompletionOr<Crypto::BigFraction> total_relative_duration(VM& vm, InternalD
 }
 
 // 7.5.40 TemporalDurationToString ( duration, precision ), https://tc39.es/proposal-temporal/#sec-temporal-temporaldurationtostring
-String temporal_duration_to_string(Duration const& duration, Precision precision)
+Utf16String temporal_duration_to_string(Duration const& duration, Precision precision)
 {
     // 1. Let sign be DurationSign(duration).
     auto sign = duration_sign(duration);
 
     // 2. Let datePart be the empty String.
-    StringBuilder date_part;
+    Utf16StringBuilder date_part;
 
     // 3. If duration.[[Years]] ≠ 0, then
     if (duration.years() != 0) {
@@ -1508,7 +1509,7 @@ String temporal_duration_to_string(Duration const& duration, Precision precision
     }
 
     // 7. Let timePart be the empty String.
-    StringBuilder time_part;
+    Utf16StringBuilder time_part;
 
     // 8. If duration.[[Hours]] ≠ 0, then
     if (duration.hours() != 0) {
@@ -1552,17 +1553,20 @@ String temporal_duration_to_string(Duration const& duration, Precision precision
     auto sign_part = sign < 0 ? "-"sv : ""sv;
 
     // 15. Let result be the string concatenation of signPart, the code unit 0x0050 (LATIN CAPITAL LETTER P) and datePart.
-    StringBuilder result;
-    result.appendff("{}P{}", sign_part, date_part.string_view());
+    Utf16StringBuilder result;
+    result.append_ascii(sign_part);
+    result.append_ascii('P');
+    result.append(date_part.view());
 
     // 16. If timePart is not the empty String, then
     if (!time_part.is_empty()) {
         // a. Set result to the string concatenation of result, the code unit 0x0054 (LATIN CAPITAL LETTER T), and timePart.
-        result.appendff("T{}", time_part.string_view());
+        result.append_ascii('T');
+        result.append(time_part.view());
     }
 
     // 17. Return result.
-    return MUST(result.to_string());
+    return result.to_string();
 }
 
 // 7.5.41 AddDurations ( operation, duration, other ), https://tc39.es/proposal-temporal/#sec-temporal-adddurations
diff --git a/Libraries/LibJS/Runtime/Temporal/Duration.h b/Libraries/LibJS/Runtime/Temporal/Duration.h
index 98f068b9ac..c10d64364c 100644
--- a/Libraries/LibJS/Runtime/Temporal/Duration.h
+++ b/Libraries/LibJS/Runtime/Temporal/Duration.h
@@ -153,7 +153,7 @@ ThrowCompletionOr<DurationNudgeResult> nudge_to_day_or_time(VM&, InternalDuratio
 ThrowCompletionOr<InternalDuration> bubble_relative_duration(VM&, i8 sign, InternalDuration, Crypto::SignedBigInteger const& nudged_epoch_ns, ISODateTime const&, Optional<String const&> time_zone, String const& calendar, Unit largest_unit, Unit smallest_unit);
 ThrowCompletionOr<InternalDuration> round_relative_duration(VM&, InternalDuration, Crypto::SignedBigInteger const& origin_epoch_ns, Crypto::SignedBigInteger const& dest_epoch_ns, ISODateTime const&, Optional<String const&> time_zone, String const& calendar, Unit largest_unit, u64 increment, Unit smallest_unit, RoundingMode);
 ThrowCompletionOr<Crypto::BigFraction> total_relative_duration(VM&, InternalDuration const&, Crypto::SignedBigInteger const& origin_epoch_ns, Crypto::SignedBigInteger const& dest_epoch_ns, ISODateTime const&, Optional<String const&> time_zone, String const& calendar, Unit);
-String temporal_duration_to_string(Duration const&, Precision);
+Utf16String temporal_duration_to_string(Duration const&, Precision);
 ThrowCompletionOr<GC::Ref<Duration>> add_durations(VM&, ArithmeticOperation, Duration const&, Value);
 
 }
diff --git a/Libraries/LibJS/Runtime/Temporal/PlainTime.cpp b/Libraries/LibJS/Runtime/Temporal/PlainTime.cpp
index 98eaa7d126..2815621d72 100644
--- a/Libraries/LibJS/Runtime/Temporal/PlainTime.cpp
+++ b/Libraries/LibJS/Runtime/Temporal/PlainTime.cpp
@@ -445,7 +445,7 @@ ThrowCompletionOr<TemporalTimeLike> to_temporal_time_record(VM& vm, Object const
 }
 
 // 4.5.13 TimeRecordToString ( time, precision ), https://tc39.es/proposal-temporal/#sec-temporal-timerecordtostring
-String time_record_to_string(Time const& time, SecondsStringPrecision::Precision precision)
+Utf16String time_record_to_string(Time const& time, SecondsStringPrecision::Precision precision)
 {
     // 1. Let subSecondNanoseconds be time.[[Millisecond]] × 10**6 + time.[[Microsecond]] × 10**3 + time.[[Nanosecond]].
     auto sub_second_nanoseconds = (static_cast<u64>(time.millisecond) * 1'000'000) + (static_cast<u64>(time.microsecond) * 1000) + static_cast<u64>(time.nanosecond);
diff --git a/Libraries/LibJS/Runtime/Temporal/PlainTime.h b/Libraries/LibJS/Runtime/Temporal/PlainTime.h
index c8b93b1e95..e5e36d8927 100644
--- a/Libraries/LibJS/Runtime/Temporal/PlainTime.h
+++ b/Libraries/LibJS/Runtime/Temporal/PlainTime.h
@@ -60,7 +60,7 @@ Time balance_time(double hour, double minute, double second, double millisecond,
 Time balance_time(double hour, double minute, double second, double millisecond, double microsecond, Crypto::SignedBigInteger const& nanosecond);
 ThrowCompletionOr<GC::Ref<PlainTime>> create_temporal_time(VM&, Time const&, GC::Ptr<FunctionObject> new_target = {});
 ThrowCompletionOr<TemporalTimeLike> to_temporal_time_record(VM&, Object const& temporal_time_like, Completeness = Completeness::Complete);
-String time_record_to_string(Time const&, SecondsStringPrecision::Precision);
+Utf16String time_record_to_string(Time const&, SecondsStringPrecision::Precision);
 i8 compare_time_record(Time const&, Time const&);
 Time add_time(Time const&, TimeDuration const& time_duration);
 Time round_time(Time const&, u64 increment, Unit, RoundingMode);
diff --git a/Libraries/LibTest/JavaScriptTestRunner.h b/Libraries/LibTest/JavaScriptTestRunner.h
index 758bdbf87c..00c80bfe10 100644
--- a/Libraries/LibTest/JavaScriptTestRunner.h
+++ b/Libraries/LibTest/JavaScriptTestRunner.h
@@ -263,7 +263,7 @@ inline ErrorOr<JsonValue> get_test_results(JS::Realm& realm)
     auto results = MUST(realm.global_object().get("__TestResults__"_utf16_fly_string));
     auto maybe_json_string = MUST(JS::JSONObject::stringify_impl(*g_vm, results, JS::js_undefined(), JS::js_undefined()));
     if (maybe_json_string.has_value())
-        return JsonValue::from_string(*maybe_json_string);
+        return JsonValue::from_string(MUST(maybe_json_string->utf16_view().to_utf8()));
     return JsonValue();
 }
 
diff --git a/Libraries/LibWeb/Crypto/SubtleCrypto.cpp b/Libraries/LibWeb/Crypto/SubtleCrypto.cpp
index 817ac662e9..8ce8df6d84 100644
--- a/Libraries/LibWeb/Crypto/SubtleCrypto.cpp
+++ b/Libraries/LibWeb/Crypto/SubtleCrypto.cpp
@@ -977,7 +977,8 @@ GC::Ref<WebIDL::Promise> SubtleCrypto::wrap_key(Bindings::KeyFormat format, GC::
             }
 
             // 2. Let bytes be the result of UTF-8 encoding json.
-            bytes = MUST(ByteBuffer::copy(maybe_json.value()->bytes()));
+            auto json = MUST(maybe_json.value()->utf16_view().to_utf8());
+            bytes = MUST(ByteBuffer::copy(json.bytes()));
         }
 
         // Otherwise: