We've recently set up cPanel on VPS to host our big Symfony2 project. We were developing locally on a MAMP setup which was really fast. For production, we moved initially to a FastCGI shared host which was fairly fast as well. But to get PHP 5.3 we needed to make a few messy fiddles we weren't that happy and they didn't have APC. So we finally moved to a VPS so we could have PHP 5.3 with mod_php and whatever other guff we needed.
However, the system was incredibly slow. Between 50% and 30% of the speed of our local development server (which wasn't anything flash). We tried a bunch of combinations of memcache and APC configurations and then tried a few different Symfony2 options but nothing made much difference. In the end it turned out to be the
open_basedir option that is normally on with cPanel accounts. This disables fstat caching, and Symfony2 does a LOT of fstat calls. Turning off
open_basedir solved the issue and reduced page generation times from about 1500ms to 600ms. Which may not be an option for everyone.