Is it required to grab v8::Locker before making v8::Function::Call?

I'm using V8 to execute some custom javascript code, exposing OnUpdate function to JS world. Overall code works fine but currently I'm concerned about performance of below code - is it required to grab v8::Locker for executing any user defined function? Instruments.app shows code here spends way too much time in v8::Locker constructor and destructor -

90 ms (in actual code execution) vs ~4000ms (by Locker & ~Locker) - this is absurd and I feel I might be doing something wrong.

So my basic question is it really necessary to grab v8::Locker to execute a v8::Function::Call? In current state if I comment out v8::LockerI get below error message:

# Fatal error in HandleScope::HandleScope
# Entering the V8 API without proper locking in place

Code snippet:

int Bucket::send_doc_update_bucket(const char *msg) {
    Locker locker(GetIsolate());
    Isolate::Scope isolate_scope(GetIsolate());
    HandleScope handle_scope(GetIsolate());

    Local<Context> context = Local<Context>::New(GetIsolate(), context_);
    Context::Scope context_scope(context);

    TryCatch try_catch;

    Local<Value> args[1];
    args[0] = String::NewFromUtf8(GetIsolate(), msg);

    assert(!try_catch.HasCaught());

    Handle<Value> val;
    if(!context->Global()->Get(context,
                               createUtf8String(GetIsolate(),
                                                "OnUpdate")).ToLocal(&val) ||
      !val->IsFunction()) {
          return 3;
    }

    Handle<Function> on_doc_update = Handle<Function>::Cast(val);
    on_doc_update->Call(context, context->Global(), 1, args);

    if (try_catch.HasCaught()) {
        //w->last_exception = ExceptionString(GetIsolate(), &try_catch);
        return 2;
    }

    return 0;
 }