In my experience, the US pre-clearance doesn't take that long. you scan your passport in one of the kiosks and give the print out to the officer. Usually they ask you a couple of questions, and they send you on your way, that is if you don't get the dreaded SSSS, in which case expect to spend 5-10 minutes extra or more. I would be more concerned with the line through security (CATSA), it can take up to an hour when it gets really busy. So I suggest you leave early.