To fix the issue with the mega menu go to Theme Options > Speed optimization > disable Ajax mega menu loading option. Clear cache and check the mega menu on the checkout page after that.
Revolution slider is heavy by itself and even 1 slider adds seconds to page loading. In your case, you added 3 sliders on the page. Why don’t you use responsive options of the revolution slider to show different layers suitable for every responsive breakpoint within the 1 slider https://youtu.be/inIULl3QolI ?