When I run certain types of activity log our server has a tendency to hang for up to about 10 minutes, sometimes to such a severe degree that I have received phone calls from users asking why Moodle is down.
This problem seems to be particularly noticeable when running a report for a single user, especially when run from within a user's profile e.g. by a teacher.
I have even observed this (although not to as great an extent) on moodle.org, using the URL http://moodle.org/course/user.php?id=5&user=153&mode=outline!
I was wondering if this was a common issue and whether anyone has any MySQL performance tuning advice which may help to alleviate this.