Dave Dash has another performance tip for Symfony users involving a simple change involving your .htaccess file.
One performance boost that can be garnered from a symfony app (or any app for that matter) is disabling .htaccess. .htaccess does not need to be parsed on each visit to your app. Disabling .htaccess is trivial in your VirtualHost or another relevant part of your apache configuration place.
The information (like routing rules) that was inside of the .htaccess is then moved back into the VirtualHost inside the Apache configuration to make for less files the Apache instance needs to open to get its job done.