Well honestly at this stage I would likely already checking the linux server logs and processes.
If you have to test this in a live environment I recommend doing so when you have very few users online, ideally none. (easier to read the logs and follow what is happening)
Clear your log.
Reload the page which is loading in 15+ seconds.
Review the log, look for issues. If you are lucky something might pop out at you. Possibly even compare with one of the other fast loading pages.
Sorry, this is really only an option if you have SSH access and at least a basic understanding of Linux.
Other Options
- Have you tested this in Moodle 2.2?
Completion tracking > Disabled... then my course loads in the expected time (approx 1 second).
I cannot reproduce this on my Moodle 2.2+ servers. All load times are very similar. With completion tracking enabled.