UI.X gives you the option to load whatever javascript based features (sticky footer, sticky navigation, off canvas sidebar, dozens others) or you could load none. By toggling off each setting, all HTML, CSS and Javascript for that feature are removed entirely. So technically, if you turned all these features off, you would have something that is as bare bones as you can get. So you could start from that and work your way up.
So in regards to heavy loading for UI.X, definitely no more than any other theme. But if you want the advanced features now or in the future, UI.X is the best of both worlds.