In reply to markk5.

The faster the request is done - the more rpm you get, pages load faster. We're not talking about CPU load but about the time it takes to serve the request. And it matters for everyone. Even if you have low load, test your site with tools.pingdom.com or similar tool and see how much time you are waiting for the response from server - usually it's 300-800ms. With HHVM it's 80-140ms. On EACH request which loads WP and PHP. I'm not talking about a full page cache served by Nginx from RAM because the FPC should really be your last resort. If you're running a site which needs users to be logged in - you can't use FPC. In this case Nginx + HHVM + Redis + MariaDB is a perfect combo to make your site lightning fast and save you some money on servers.