UI animations have a lot to do with it, but I'm more referring to UI jank over simple stuff like 'enumerating a list of sharing targets' or 'swiping to the next page of the launchboard' when the phone is otherwise idle. Or, my favorites: the multi-second pause from unlocking my phone during a phone/skype call, or the taking of an actual eternity just to answer said fucking Skype call, of which half the time the call has already hung up once my phone is finally responding...