Yep, Custom JS code work only upon widget’s installation, not in the preview mode. This is why you haven’t noticed any changes when checking from the configurator 
As for the idea to reduce the response time, I’ve added it to the Wishlist on your behalf - Reduce response time.
I am afraid, we can’t give any promises for now, since we completely depend on our provider in this case. However, if they have any changes on their end, we’ll try to implement them to the widget too