With 100 simultaneous users taking a 60 question quiz with audio and images, our school's Moodle slowed down to a crawl, giving everyone spinning balls on the browser for 2-3 minutes to refresh a page.
Our LAMP server monitoring software showed no problems anywhere--loads were minor. No client problems, no Moodle problem, and no network problem. It was a mystery until we turned off the load balancer (thus running moodle and its database from one server alone). Then we had no problems administering the quiz, even up to 200 simultaneous users on the same quiz the next day.
Why is the load balancer causing the problem? That we do not know. We do suspect that our settings and design could be the problem because we built the load balancer ourselves to save money, using open source Pacemaker, Ultramonkey, and Pound.
If anyone has experience with building an open source load balancer, we would like to know what worked for your situation and how we can avoid slowdowns.