APC
Make sure you have APC installed on your server. It won't work without it. It's easy to install, so if you don't have root acces to your server, ask your hosting provider to take care of it for you.
Installation instructions: Linux | Windows


Important: Make sure to include apc.rfc1867 = on in your php.ini file after APC is installed. Your php.ini file should be located at /etc/php.ini.