A bytecode cache like APC will cache the bytecode that is generated by php prior to executing it. That means that the parsing of a file and the creation of the bytecode happens only once and not on every request. This is especially useful when using large libraries and/or frameworks. Together with file bundling for production this should give you a significant performance improvement. To get the most out of a bytecode cache you should contact the manual pages since most of these caches have a lot of configuration options which you can tweak to optimize the cache to your needs.