[smacktoward]

I have been using Piwik/Matomo for years, and have never experienced this.

That being said, ultimately it's a frontend on a MySQL database, so there's lots of ways it could theoretically be slow -- MySQL isn't configured/tuned properly, the database server isn't resourced appropriately for the amount of data it's hosting, etc. But this is going to be the case for any self-hosted solution.