So far, the site seems slightly snappier now that I’ve replaced the venerable (but old and unmaintained) APC with APCu for user-space object caching and ZendOpcache for opcode caching. Various people report seeing 10-30% improvement in speed with the new opcode cache / optimizer that will be the default in PHP 5.5. Also APCu is nice because it looks exactly the same to existing apps & requires no configuration. One thing I recall reading about APCu, however, is that it doesn’t provide the upload progress function that APC had. There are alternatives for that, though.