I also reported this in mooodle forum:
Basically if you have a big class, say 1000+ students in the course, when you set the no. of students per page to over 100, the page would load VERY slowly. Here is a case that I have when set the number to 100:
9.033779 secs Included 64 files DB queries 200 Record cache hit/miss ratio : 0/0
Looks like some optimization need to be done.