It's not a race versus somebody else. It's a time to show you're making progress. So during the round you don't really feel rushed but when you find out you were ~average or ahead of the pack....or way behind.
The weights are light to begin with. But the sets/reps are crazy. Each exercise has something like: Press lifts 185/125/80 which is 185 for men, 125 for women or 80 for beginners. For form the intructor is on you like stink if you start to slack off. There's usually around 12 people in a class and since people "rotate" exercises the instructor has plenty of time to walk around and give pointers. They also do a good job of demonstrating each exercise at the beginning of class. To be CF instructors they go through a pretty comprehensive training program, so at least there's a basic standard to the instructors (the gyms can vary big time tho). Many of the exercises are legs related, crossfit.com has a ton of demo vids.
It works. There's some dudes that look like they could be in the movie 300. I was picking up a 95lb bar, this guy was picking up a 240lb bar. The bar was bowing when he'd pick it up. But everybody is supportive.
Try out a class. I'm hooked!