I think it depends. There have been times where I've seen someone finish LONG exams (10 math problems; 2 hours allotted) so fast (finished in 15 minutes) that you know they failed. However, I tend to finish exams first or before more than 90% of the class and usually do pretty well on them. I think my comm professor said it depends on your personality, and that some people will just finish exams fast regardless of how hard it is, but I disagree. I think it depends on how comfortable you are with the subject matter (which probably has a lot to do with how much you studied).