I've recently been informed by my host that my site is causing very high server load, although I don't really know how I can find out what is actually causing it. I tested to make sure and yes, its my site (and my coding).

The speed of the scripts is fine, and I have optimised all of 20 or so mySQL queries to less than 0.001 seconds. I use mod_rewrite quite a lot too, but I don't see how this could cause such a rise in server load.

Is there any way of finding out whether a certain function or operation is causing the high load?