Improve performance of a WordPress website (VPS/Centos/cPanel)

I have a Linux VPS running cPanel which hosts just one WordPress website. The server is quite powerful with 6 core processors and 20GB RAM and yet the website runs fairly slowly , taking about 5 seconds to load a standard page. The website, [login to view URL] runs the latest version of WordPress and has a large number of plugins (all used). The web server has only recently been setup and runs the latest stable releases of Cpanel, Apache, PHP (5.4.16), mySQL (5.5.32) etc. On top of the normal CPanel/WHM install I have added some Configserver plugins for firewall and intrusion detection, as well as recently install Munin to monitor performance. I need some one with experience of Linus, WHM and WordPress hosting who can assess the environment and let me know where problems might lie and what expectations I should have for improved performance. Please include the word monkey in the your communication to show you have read this. Then provide recommendations and carry out changes in a controlled manor. I do have a copy of the website running on a similarly configured web server that could be used for testing purposes. Perhaps the answer is some sort of server caching tool like APC alongside a WordPress plugin like [login to view URL] Happy bidding!