diff --git a/src/heap/factory.cc b/src/heap/factory.cc
index 3baa5fdfc8..6b5d320737 100644
--- a/src/heap/factory.cc
+++ b/src/heap/factory.cc
@@ -736,8 +736,16 @@ Handle<String> Factory::InternalizeOneByteString(
 }
 
 Handle<String> Factory::InternalizeTwoByteString(
-    const Vector<const uc16>& string) {
-  TwoByteStringKey key(string, HashSeed(isolate()));
+    Handle<SeqTwoByteString> string, int from, int length,
+    bool convert_to_one_byte) {
+  SeqTwoByteSubStringKey key(isolate(), string, from, length,
+                             convert_to_one_byte);
+  return InternalizeStringWithKey(&key);
+}
+
+Handle<String> Factory::InternalizeTwoByteString(
+    const Vector<const uc16>& string, bool convert_to_one_byte) {
+  TwoByteStringKey key(string, HashSeed(isolate()), convert_to_one_byte);
   return InternalizeStringWithKey(&key);
 }
 
@@ -924,22 +932,29 @@ Handle<SeqOneByteString> Factory::AllocateRawOneByteInternalizedString(
 
 Handle<String> Factory::AllocateTwoByteInternalizedString(
     const Vector<const uc16>& str, uint32_t hash_field) {
-  CHECK_GE(String::kMaxLength, str.length());
-  DCHECK_NE(0, str.length());  // Use Heap::empty_string() instead.
-
-  Map map = *internalized_string_map();
-  int size = SeqTwoByteString::SizeFor(str.length());
-  HeapObject result =
-      AllocateRawWithImmortalMap(size, AllocationType::kOld, map);
-  Handle<SeqTwoByteString> answer(SeqTwoByteString::cast(result), isolate());
-  answer->set_length(str.length());
-  answer->set_hash_field(hash_field);
-  DCHECK_EQ(size, answer->Size());
+  Handle<SeqTwoByteString> result =
+      AllocateRawTwoByteInternalizedString(str.length(), hash_field);
   DisallowHeapAllocation no_gc;
 
   // Fill in the characters.
-  MemCopy(answer->GetChars(no_gc), str.begin(), str.length() * kUC16Size);
+  MemCopy(result->GetChars(no_gc), str.begin(), str.length() * kUC16Size);
 
+  return result;
+}
+
+Handle<SeqTwoByteString> Factory::AllocateRawTwoByteInternalizedString(
+    int length, uint32_t hash_field) {
+  CHECK_GE(String::kMaxLength, length);
+  DCHECK_NE(0, length);  // Use Heap::empty_string() instead.
+
+  Map map = *internalized_string_map();
+  int size = SeqTwoByteString::SizeFor(length);
+  HeapObject result =
+      AllocateRawWithImmortalMap(size, AllocationType::kOld, map);
+  Handle<SeqTwoByteString> answer(SeqTwoByteString::cast(result), isolate());
+  answer->set_length(length);
+  answer->set_hash_field(hash_field);
+  DCHECK_EQ(size, result.Size());
   return answer;
 }
 
@@ -991,17 +1006,6 @@ Handle<String> Factory::NewOneByteInternalizedString(
   return result;
 }
 
-Handle<String> Factory::NewOneByteInternalizedSubString(
-    Handle<SeqOneByteString> string, int offset, int length,
-    uint32_t hash_field) {
-  Handle<SeqOneByteString> result =
-      AllocateRawOneByteInternalizedString(length, hash_field);
-  DisallowHeapAllocation no_allocation;
-  MemCopy(result->GetChars(no_allocation),
-          string->GetChars(no_allocation) + offset, length);
-  return result;
-}
-
 Handle<String> Factory::NewTwoByteInternalizedString(
     const Vector<const uc16>& str, uint32_t hash_field) {
   return AllocateTwoByteInternalizedString(str, hash_field);
diff --git a/src/heap/factory.h b/src/heap/factory.h
index bc8cd14b94..59d8f52e6d 100644
--- a/src/heap/factory.h
+++ b/src/heap/factory.h
@@ -245,8 +245,12 @@ class V8_EXPORT_PRIVATE Factory {
   Handle<String> InternalizeOneByteString(const Vector<const uint8_t>& str);
   Handle<String> InternalizeOneByteString(Handle<SeqOneByteString>, int from,
                                           int length);
+  Handle<String> InternalizeTwoByteString(Handle<SeqTwoByteString>, int from,
+                                          int length,
+                                          bool convert_to_one_byte = false);
 
-  Handle<String> InternalizeTwoByteString(const Vector<const uc16>& str);
+  Handle<String> InternalizeTwoByteString(const Vector<const uc16>& str,
+                                          bool convert_to_one_byte = false);
 
   template <class StringTableKey>
   Handle<String> InternalizeStringWithKey(StringTableKey* key);
@@ -318,13 +322,15 @@ class V8_EXPORT_PRIVATE Factory {
   Handle<String> NewOneByteInternalizedString(const Vector<const uint8_t>& str,
                                               uint32_t hash_field);
 
-  Handle<String> NewOneByteInternalizedSubString(
-      Handle<SeqOneByteString> string, int offset, int length,
-      uint32_t hash_field);
+  Handle<SeqOneByteString> AllocateRawOneByteInternalizedString(
+      int length, uint32_t hash_field);
 
   Handle<String> NewTwoByteInternalizedString(const Vector<const uc16>& str,
                                               uint32_t hash_field);
 
+  Handle<SeqTwoByteString> AllocateRawTwoByteInternalizedString(
+      int length, uint32_t hash_field);
+
   Handle<String> NewInternalizedStringImpl(Handle<String> string, int chars,
                                            uint32_t hash_field);
 
@@ -1068,9 +1074,6 @@ class V8_EXPORT_PRIVATE Factory {
   Handle<String> AllocateInternalizedStringImpl(T t, int chars,
                                                 uint32_t hash_field);
 
-  Handle<SeqOneByteString> AllocateRawOneByteInternalizedString(
-      int length, uint32_t hash_field);
-
   Handle<String> AllocateTwoByteInternalizedString(
       const Vector<const uc16>& str, uint32_t hash_field);
 
diff --git a/src/json/json-parser.cc b/src/json/json-parser.cc
index ea8837423e..0949e18bdf 100644
--- a/src/json/json-parser.cc
+++ b/src/json/json-parser.cc
@@ -964,15 +964,16 @@ namespace {
 
 template <typename Char>
 bool Matches(const Vector<const Char>& chars, Handle<String> string) {
-  if (string.is_null()) return false;
+  DCHECK(!string.is_null());
 
-  // Only supports internalized strings in their canonical representation (one
-  // byte encoded as two-byte will return false here).
-  if ((sizeof(Char) == 1) != string->IsOneByteRepresentation()) return false;
   if (chars.length() != string->length()) return false;
 
   DisallowHeapAllocation no_gc;
-  const Char* string_data = string->GetChars<Char>(no_gc);
+  if (string->IsOneByteRepresentation()) {
+    const uint8_t* string_data = string->GetChars<uint8_t>(no_gc);
+    return CompareChars(chars.begin(), string_data, chars.length()) == 0;
+  }
+  const uint16_t* string_data = string->GetChars<uint16_t>(no_gc);
   return CompareChars(chars.begin(), string_data, chars.length()) == 0;
 }
 
@@ -991,7 +992,7 @@ Handle<String> JsonParser<Char>::DecodeString(
       DecodeString(dest, string.start(), string.length());
     } else {
       DCHECK_IMPLIES(string.internalize(), string.needs_conversion());
-      i::CopyChars(dest, chars_ + string.start(), string.length());
+      CopyChars(dest, chars_ + string.start(), string.length());
     }
 
     Vector<const SinkChar> data(dest, string.length());
@@ -1013,13 +1014,6 @@ Handle<String> JsonParser<Char>::MakeString(const JsonString& string,
   if (string.length() == 0) return factory()->empty_string();
 
   if (sizeof(Char) == 1) {
-    if (V8_UNLIKELY(string.needs_conversion())) {
-      DCHECK(string.has_escape());
-      Handle<SeqTwoByteString> intermediate =
-          factory()->NewRawTwoByteString(string.length()).ToHandleChecked();
-      return DecodeString<uint16_t>(string, intermediate, hint);
-    }
-
     if (string.internalize() && !string.has_escape()) {
       if (!hint.is_null()) {
         Vector<const Char> data(chars_ + string.start(), string.length());
@@ -1035,12 +1029,13 @@ Handle<String> JsonParser<Char>::MakeString(const JsonString& string,
           Vector<const uint8_t>::cast(chars));
     }
 
-    Handle<SeqOneByteString> intermediate =
-        factory()->NewRawOneByteString(string.length()).ToHandleChecked();
-    return DecodeString<uint8_t>(string, intermediate, hint);
-  }
+    if (V8_UNLIKELY(string.needs_conversion())) {
+      DCHECK(string.has_escape());
+      Handle<SeqTwoByteString> intermediate =
+          factory()->NewRawTwoByteString(string.length()).ToHandleChecked();
+      return DecodeString<uint16_t>(string, intermediate, hint);
+    }
 
-  if (string.needs_conversion()) {
     Handle<SeqOneByteString> intermediate =
         factory()->NewRawOneByteString(string.length()).ToHandleChecked();
     return DecodeString<uint8_t>(string, intermediate, hint);
@@ -1052,13 +1047,19 @@ Handle<String> JsonParser<Char>::MakeString(const JsonString& string,
       if (Matches(data, hint)) return hint;
     }
     if (chars_may_relocate_) {
-      Handle<String> substring = factory()->NewProperSubString(
-          source_, string.start(), string.start() + string.length());
-      return factory()->InternalizeString(substring);
+      return factory()->InternalizeTwoByteString(
+          Handle<SeqTwoByteString>::cast(source_), string.start(),
+          string.length(), string.needs_conversion());
     }
     Vector<const Char> chars(chars_ + string.start(), string.length());
     return factory()->InternalizeTwoByteString(
-        Vector<const uint16_t>::cast(chars));
+        Vector<const uint16_t>::cast(chars), string.needs_conversion());
+  }
+
+  if (string.needs_conversion()) {
+    Handle<SeqOneByteString> intermediate =
+        factory()->NewRawOneByteString(string.length()).ToHandleChecked();
+    return DecodeString<uint8_t>(string, intermediate, hint);
   }
 
   Handle<SeqTwoByteString> intermediate =
diff --git a/src/objects.cc b/src/objects.cc
index fab575b646..8042a6e399 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -6790,6 +6790,8 @@ template Handle<String> StringTable::LookupKey(Isolate* isolate,
                                                TwoByteStringKey* key);
 template Handle<String> StringTable::LookupKey(Isolate* isolate,
                                                SeqOneByteSubStringKey* key);
+template Handle<String> StringTable::LookupKey(Isolate* isolate,
+                                               SeqTwoByteSubStringKey* key);
 
 Handle<String> StringTable::AddKeyNoResize(Isolate* isolate,
                                            StringTableKey* key) {
diff --git a/src/objects/string-inl.h b/src/objects/string-inl.h
index 7868575772..47e79ff2c5 100644
--- a/src/objects/string-inl.h
+++ b/src/objects/string-inl.h
@@ -197,13 +197,17 @@ Char FlatStringReader::Get(int index) {
 template <typename Char>
 class SequentialStringKey final : public StringTableKey {
  public:
-  SequentialStringKey(const Vector<const Char>& chars, uint64_t seed)
+  SequentialStringKey(const Vector<const Char>& chars, uint64_t seed,
+                      bool convert = false)
       : SequentialStringKey(StringHasher::HashSequentialString<Char>(
                                 chars.begin(), chars.length(), seed),
-                            chars) {}
+                            chars, convert) {}
 
-  SequentialStringKey(int hash, const Vector<const Char>& chars)
-      : StringTableKey(hash, chars.length()), chars_(chars) {}
+  SequentialStringKey(int hash, const Vector<const Char>& chars,
+                      bool convert = false)
+      : StringTableKey(hash, chars.length()),
+        chars_(chars),
+        convert_(convert) {}
 
   bool IsMatch(String s) override {
     DisallowHeapAllocation no_gc;
@@ -226,12 +230,14 @@ class SequentialStringKey final : public StringTableKey {
 
  private:
   Vector<const Char> chars_;
+  bool convert_;
 };
 
 using OneByteStringKey = SequentialStringKey<uint8_t>;
 using TwoByteStringKey = SequentialStringKey<uint16_t>;
 
-class SeqOneByteSubStringKey final : public StringTableKey {
+template <typename Char>
+class SeqSubStringKey final : public StringTableKey {
  public:
 // VS 2017 on official builds gives this spurious warning:
 // warning C4789: buffer 'key' of size 16 bytes will be overrun; 4 bytes will
@@ -241,9 +247,13 @@ class SeqOneByteSubStringKey final : public StringTableKey {
 #pragma warning(push)
 #pragma warning(disable : 4789)
 #endif
-  SeqOneByteSubStringKey(Isolate* isolate, Handle<SeqOneByteString> string,
-                         int from, int len)
-      : StringTableKey(0, len), string_(string), from_(from) {
+  SeqSubStringKey(Isolate* isolate,
+                  Handle<typename CharTraits<Char>::String> string, int from,
+                  int len, bool convert = false)
+      : StringTableKey(0, len),
+        string_(string),
+        from_(from),
+        convert_(convert) {
     // We have to set the hash later.
     DisallowHeapAllocation no_gc;
     uint32_t hash = StringHasher::HashSequentialString(
@@ -252,7 +262,8 @@ class SeqOneByteSubStringKey final : public StringTableKey {
 
     DCHECK_LE(0, length());
     DCHECK_LE(from_ + length(), string_->length());
-    DCHECK(string_->IsSeqOneByteString());
+    DCHECK_EQ(string_->IsSeqOneByteString(), sizeof(Char) == 1);
+    DCHECK_EQ(string_->IsSeqTwoByteString(), sizeof(Char) == 2);
   }
 #if defined(V8_CC_MSVC)
 #pragma warning(pop)
@@ -270,15 +281,33 @@ class SeqOneByteSubStringKey final : public StringTableKey {
   }
 
   Handle<String> AsHandle(Isolate* isolate) override {
-    return isolate->factory()->NewOneByteInternalizedSubString(
-        string_, from_, length(), hash_field());
+    if (sizeof(Char) == 1 || (sizeof(Char) == 2 && convert_)) {
+      Handle<SeqOneByteString> result =
+          isolate->factory()->AllocateRawOneByteInternalizedString(
+              length(), hash_field());
+      DisallowHeapAllocation no_gc;
+      CopyChars(result->GetChars(no_gc), string_->GetChars(no_gc) + from_,
+                length());
+      return result;
+    }
+    Handle<SeqTwoByteString> result =
+        isolate->factory()->AllocateRawTwoByteInternalizedString(length(),
+                                                                 hash_field());
+    DisallowHeapAllocation no_gc;
+    CopyChars(result->GetChars(no_gc), string_->GetChars(no_gc) + from_,
+              length());
+    return result;
   }
 
  private:
-  Handle<SeqOneByteString> string_;
+  Handle<typename CharTraits<Char>::String> string_;
   int from_;
+  bool convert_;
 };
 
+using SeqOneByteSubStringKey = SeqSubStringKey<uint8_t>;
+using SeqTwoByteSubStringKey = SeqSubStringKey<uint16_t>;
+
 bool String::Equals(String other) {
   if (other == *this) return true;
   if (this->IsInternalizedString() && other->IsInternalizedString()) {