There is one timed quest that I recall (if you agree to be an arsonist, you have to do so that day). There is a quest to arrange an escort for some healers, which needs to be done before completing a couple of other major quests, otherwise they will move by themselves, and you will not get the experience. Another quest in the first town has to be done before the healers move.
Experience points for completing a quest are higher at higher character levels. This is opposite to killing monsters, where the experience decreases if you are at a much higher level than your opponent.
A couple quests can not be solved, and will fail as part of the plot no matter how fast you are. There are other opposing quests where you must choose which side to help, failing the opposite quest.
