I've noticed the opposite. If I scroll through the saves on the stock M$ dash it bout crashes at points if the game has more then like 5 saves. UIX handles it just fine.
As for the transitions, it might be the veiwpoints are further apart. (where the camera is and looks and sees to) But if you mean how smooth the main menu moves from tab to tab and such, that was actually done on purpose. Once you get used to the smoothness of it the other dashes seem jerky. Other then that, uuuh... nothing comes to mind.