Eagerly follow transitions to existing maps while json parsing.

src/json-parser.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 271 matching lines @@
       return ParseJsonArray();
     default:
       return ReportUnexpectedCharacter();
   }
 }
 
 
 // Parse a JSON object. Position must be right at '{'.
 template <bool seq_ascii>
 Handle<Object> JsonParser<seq_ascii>::ParseJsonObject() {
+  int current_index = 0;
+  Handle<Object> prototype;
   Handle<JSFunction> object_constructor(
       isolate()->native_context()->object_function());
   Handle<JSObject> json_object =
       isolate()->factory()->NewJSObject(object_constructor);
   ASSERT_EQ(c0_, '{');
 
   AdvanceSkipWhitespace();
   if (c0_ != '}') {
     do {
       if (c0_ != '"') return ReportUnexpectedCharacter();
       Handle<String> key = ParseJsonSymbol();
       if (key.is_null() || c0_ != ':') return ReportUnexpectedCharacter();
       AdvanceSkipWhitespace();
       Handle<Object> value = ParseJsonValue();
       if (value.is_null()) return ReportUnexpectedCharacter();
 
       uint32_t index;
       if (key->AsArrayIndex(&index)) {
         JSObject::SetOwnElement(json_object, index, value, kNonStrictMode);
       } else if (key->Equals(isolate()->heap()->Proto_symbol())) {
-        SetPrototype(json_object, value);
+        prototype = value;
       } else {
-        JSObject::SetLocalPropertyIgnoreAttributes(
-            json_object, key, value, NONE);
+        if (json_object->TryTransitionToField(*key)) {
+          json_object->FastPropertyAtPut(current_index++, *value);
+        } else {
+          JSObject::SetLocalPropertyIgnoreAttributes(
+              json_object, key, value, NONE);
+        }
       }
     } while (MatchSkipWhiteSpace(','));
     if (c0_ != '}') {
       return ReportUnexpectedCharacter();
     }
+    if (!prototype.is_null()) SetPrototype(json_object, prototype);
   }
   AdvanceSkipWhitespace();
   return json_object;
 }
 
 // Parse a JSON array. Position must be right at '['.
 template <bool seq_ascii>
 Handle<Object> JsonParser<seq_ascii>::ParseJsonArray() {
   ZoneScope zone_scope(zone(), DELETE_ON_EXIT);
   ZoneList<Handle<Object> > elements(4, zone());
@@ skipping 311 matching lines @@
   }
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_