Sorry, took me a while to get to this but I think the loading times aren't too bad, as IJ said it's not much compared to other in-browser game engines.
The full-colour dithered art style does tend result in decks being on the bigger size in my experience. It sounds like you're on the right track in terms of keeping the size down though - basically, avoiding duplicating the same backgrounds is what should help the best. So using canvas widgets to add elements without having to duplicate the rest of the background, or enabling/disabling buttons depending on what interactions you should be able to do and such.