With limited computing power, the user interface strives to be as fast as possible, trying to establish real-time communication with the user. Every second that the user has to wait is a reminder that the user deals with technology, not with a real, natural product. So the UI needs to be real-time, if possible.
Jakob Nielsen from the Nielsen Norman Group (remember that I mentioned Don Norman?) states a good example of when the UI is too fast so that user get confused and eventually irritated. It’s worth a thought.