I feel GRE is the most diffiicult exam I have taken because of its extreme short test time. The test questions are not difficult, especially the math is too easy compared to the math I have studied in my graduate school. But the problem is that I just can't finish the test in its given time limit after trying many times. I don't know the cause of the problem is. Probably I don't do those numberical calculations often since undergraduate school (since undergraduate school we mainly engage in algebra and am rarely required to work out the numbers; usually the professors paid attention to the calculation process instead of the result, so even you miss out a sign or number and thus have the wrong result, the professor would also give you a big proportion of the score if your whole process shows the correct derivation) or probably I have never been trained to finish a test in that short time since childhood (iI have never taken a test in which I feel the time is that insuffcient as GRE; in my graduate school some of our professors even gave no restrict time limit for the exam and extended the time when they saw the need, though the difficulty level of the exam is often high).