what is it about the real thing that makes it go so much longer? are you guys just taking more time to avoid mistakes, or ..?
I agree with the previous posters - I finished early on UW blocks, but during the real thing it was closer to the wire.
To answer your question, I think it's a few things:
1) Length of passages, diagrams, figures, pictures - they tended to be a little longer
2) Agonizing over each question - during UW I think there's a bigger tendency to say...."eh...**** it, I'm going with B" whereas on Step I you're like...."crap, crap, crap, there must be something I'm missing.
3) Reviewing questions - I tended to want to review more (i.e. I'd try to flip through all 48, the ones I didn't mark I'd just glance at, the ones I did mark I'd try to reread and rethink, etc. which takes time, for the reason referred to by #2 above)
4) Fatigue - by the later blocks, you start getting fried and start having to reread sentences like 3 times to absorb it. I don't know why that is, but that was my experience.
That said, I agree with the others saying you're on the right track. If timing isn't an issue for you, go to Tutor mode. I recommend Tutor mode, because the blocks actually go faster, and you get more out of the explanations because the question and your thought-process are still fresh in your mind.