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

src/api.cc

 // Copyright 2012 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.
 
 #include "src/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 3640 matching lines @@
     name = i::Handle<i::Name>::cast(key);
   } else {
     // Call-back into JavaScript to convert the key to a string.
     i::Handle<i::Object> converted;
     if (!i::Execution::ToString(isolate, key).ToHandle(&converted)) {
       return i::MaybeHandle<i::Object>();
     }
     name = i::Handle<i::String>::cast(converted);
   }
 
-  if (name->IsString()) {
-    name = i::String::Flatten(i::Handle<i::String>::cast(name));
-  }
   return i::JSReceiver::DeleteProperty(receiver, name, language_mode);
 }
 
 }  // namespace
 
 
 MaybeLocal<Value> v8::Object::Get(Local<v8::Context> context,
                                   Local<Value> key) {
   PREPARE_FOR_EXECUTION(context, "v8::Object::Get()", Value);
   auto self = Utils::OpenHandle(this);
@@ skipping 4725 matching lines @@
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 }  // namespace internal
 }  // namespace v8