If you have a lot of traffic, we’d advise you to install Matomo On-Premise separately and use it in combination with our WordPress integration plugin WP-Matomo. There’s no specific traffic threshold we can give you on when it’s better to use Matomo On-Premise. It really depends on your server. We also need to await feedback from this beta test.
We reckon if you have more than 50,000 page views a month, you may want to think about using Matomo On-Premise with WP-Matomo instead, but this is just an estimate. In general, if the load on your server is already quite high, then it might be better to install Matomo on a separate server. See also recommended server sizing for running Matomo.