We have tried using piwik for meassurement of our website, we have about 3000 simultaneous clients to work on our system. We experienced heavy load on our server using piwik (it almost crashed). the perfomance tipps on FAQ Archive - Analytics Platform - Matomo section did not really help with the issues. Main thing was more then 400 connections on mysql and apache was on limit with 400 max clients,too.
Would be great, if anyone has any idea about optimizing. If you need further info, ask for those.
We use this hardware setup:
INTEL Core 2 Duo