The biggest single aid for me to investigating issues with Moodle is Site Administration -> Server -> Debugging
- Debug message: DEVELOPER
- Display debug messages: checked
- XML strict headers: unchecked
The main Moodle site at my school is on a Windows machine, so if I suspect a heavy server load, I login to it remotely during class and monitor the CPU usage via the Windows Task Manager. Probably UNIX has some other way to monitor the CPU.
On a commercially hosted Moodle site I maintain, I can monitor CPU usage via the control panel, Plesk. This control panel also gives me access to the Apache and PHP error logs, as well as giving me a report of when the server was maxed out on memory, CPU or disk space. These aids were very useful to me when I first took control of the server, as the Apache server had been setup with everything enabled and was demanding a lot of memory. I trimmed down the Apache modules and can now run QuizPort comfortably with 100+ simultaneous users, where initially we were struggling to cope with 40.
What you monitor and how you monitor it will depend rather on your individual setup, your expertise, and the support available from people you are connected to.
When you are ready to try it out, you can download the QuizPort module from here: