Yes, I worded that poorly. I meant loading Piwik's UI; it was cripplingly slow and often timed out. I had little luck in narrowing down the cause, but it was hardly a thorough investigation on my part, IIRC.
Assuming that your disks, DB, CPUs and RAM were all ok, maybe you did not set up PiWik's jobs that are supposed to aggregate the metrics every XX hours/days... ?
https://matomo.org/docs/setup-auto-archiving/