Make sure to flatten names before lookup. Lookup using cons strings is really slow.

src/lookup.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_LOOKUP_H_
 #define V8_LOOKUP_H_
 
 #include "src/factory.h"
 #include "src/isolate.h"
 #include "src/objects.h"
@@ skipping 34 matching lines @@
   };
 
   LookupIterator(Handle<Object> receiver, Handle<Name> name,
                  Configuration configuration = DEFAULT)
       : configuration_(ComputeConfiguration(configuration, name)),
         state_(NOT_FOUND),
         exotic_index_state_(ExoticIndexState::kUninitialized),
         interceptor_state_(InterceptorState::kUninitialized),
         property_details_(PropertyDetails::Empty()),
         isolate_(name->GetIsolate()),
-        name_(name),
+        name_(Name::Flatten(name)),
         // kMaxUInt32 isn't a valid index.
         index_(kMaxUInt32),
         receiver_(receiver),
         holder_(GetRoot(receiver_, isolate_)),
         holder_map_(holder_->map(), isolate_),
         initial_holder_(holder_),
         number_(DescriptorArray::kNotFound) {
 #ifdef DEBUG
     uint32_t index;  // Assert that the name is not an array index.
     DCHECK(!name->AsArrayIndex(&index));
 #endif  // DEBUG
     Next();
   }
 
   LookupIterator(Handle<Object> receiver, Handle<Name> name,
                  Handle<JSReceiver> holder,
                  Configuration configuration = DEFAULT)
       : configuration_(ComputeConfiguration(configuration, name)),
         state_(NOT_FOUND),
         exotic_index_state_(ExoticIndexState::kUninitialized),
         interceptor_state_(InterceptorState::kUninitialized),
         property_details_(PropertyDetails::Empty()),
         isolate_(name->GetIsolate()),
-        name_(name),
+        name_(Name::Flatten(name)),
         // kMaxUInt32 isn't a valid index.
         index_(kMaxUInt32),
         receiver_(receiver),
         holder_(holder),
         holder_map_(holder_->map(), isolate_),
         initial_holder_(holder_),
         number_(DescriptorArray::kNotFound) {
 #ifdef DEBUG
     uint32_t index;  // Assert that the name is not an array index.
     DCHECK(!name->AsArrayIndex(&index));
@@ skipping 38 matching lines @@
         initial_holder_(holder_),
         number_(DescriptorArray::kNotFound) {
     // kMaxUInt32 isn't a valid index.
     DCHECK_NE(kMaxUInt32, index_);
     Next();
   }
 
   static LookupIterator PropertyOrElement(
       Isolate* isolate, Handle<Object> receiver, Handle<Name> name,
       Configuration configuration = DEFAULT) {
+    name = Name::Flatten(name);
     uint32_t index;
     LookupIterator it =
         name->AsArrayIndex(&index)
             ? LookupIterator(isolate, receiver, index, configuration)
             : LookupIterator(receiver, name, configuration);
     it.name_ = name;
     return it;
   }
 
   static LookupIterator PropertyOrElement(
       Isolate* isolate, Handle<Object> receiver, Handle<Name> name,
       Handle<JSReceiver> holder, Configuration configuration = DEFAULT) {
+    name = Name::Flatten(name);
     uint32_t index;
     LookupIterator it =
         name->AsArrayIndex(&index)
             ? LookupIterator(isolate, receiver, index, holder, configuration)
             : LookupIterator(receiver, name, holder, configuration);
     it.name_ = name;
     return it;
   }
+
   Isolate* isolate() const { return isolate_; }
   State state() const { return state_; }
 
   Handle<Name> name() const {
     DCHECK(!IsElement());
     return name_;
   }
   Handle<Name> GetName() {
     if (name_.is_null()) {
       DCHECK(IsElement());
@@ skipping 137 matching lines @@
   Handle<JSReceiver> holder_;
   Handle<Map> holder_map_;
   const Handle<JSReceiver> initial_holder_;
   uint32_t number_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_LOOKUP_H_