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

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 8e6c554f1dddab89b225a483f841b2d66a719e34..560dc34b3004860c59c24f2c60e10c0bef7ce342 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -3658,9 +3658,6 @@ i::MaybeHandle<i::Object> DeleteObjectProperty(
     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);
 }