I think when I was doing my UWorld in systems it was much faster, especially if I was being super selective with the topics (e.g. doing a whole block of just physiology, or just pathology). When you do the questions in a random mode you will be getting some stuff that you may not have seen in a few days or more and it might take a little thinking to bring that info to the surface of your conscious thinking. n = 1 of course, but I usually finish UWorld with 10-20 minutes to spare and then go through flagged questions leisurely (random and unused only). When I do NBMEs I am usually using almost every minute available though :S