I setup one of my smaller VPS's today. Single thread running NGINX/PHP-FPM & Percona. Installing Addon's I reached 99.8% a few times. Quite often 40 to 50%. There is a TON of parsing and writing in a short burst of time. The more theme's you have the more work it has to do when installing an addon. But I would think it a bit normal. Sure it could be slown down to keep the load down but whats the point in that The rest of the time the board rarely taxes the server at all.