So we have been using Piwik for a little over a year now. Never had any issues, everything was fast, and ran great.
Had a small hiccup during the upgrade but was able to make it through. After the upgrade was complete I noticed it was a bit sliggush on the main panel. Changing to monthly stats would completely die out initially due to execution time exceeded.
I went ahead and upgraded to latest beta but it is still very sluggish compared to version 1
We are running standard Cpanel Centos 6.4, Apache 2.2.26, PHP5.4.25