KenB - you probably don't want to run xDebug on a production server. It slows down the server a lot because it has to write data with every single function call.
You might need to bump up your script execution timeout limits and the memory available to PHP as well.
Still, you'll get more data than you can imagine, and yet easy to view and understand. Quite enlightening.
adrian - look into using output buffering in PHP to roll your own simple cache. at the very least, I think it's a good thing to have set up for your most popular pages.