Convert object to fast mode after creation. That enables more optimizations....

src/apinatives.js

 // Copyright 2006-2008 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 33 matching lines @@
 function Instantiate(data, name) {
   if (!%IsTemplate(data)) return data;
   var tag = %GetTemplateField(data, kApiTagOffset);
   switch (tag) {
     case kFunctionTag:
       return InstantiateFunction(data, name);
     case kNewObjectTag:
       var Constructor = %GetTemplateField(data, kApiConstructorOffset);
       var result = Constructor ? new (Instantiate(Constructor))() : {};
       ConfigureTemplateInstance(result, data);
+      result = %ToFastProperties(result);
       return result;
     default:
       throw 'Unknown API tag <' + tag + '>';
   }
 }
 
 
 function InstantiateFunction(data, name) {
   // We need a reference to kApiFunctionCache in the stack frame
   // if we need to bail out from a stack overflow.
@@ skipping 36 matching lines @@
         var prop_data = properties[i + 2];
         var attributes = properties[i + 3];
         var value = Instantiate(prop_data, name);
         %SetProperty(obj, name, value, attributes);
       }
     } finally {
       if (requires_access_checks) %EnableAccessChecks(obj);
     }
   }
 }