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 Connect 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 servers specifications.
We recommend if you have more than 50,000 page views a month, you may want to think about using Matomo On-Premise with the Connect Matomo (WP-Matomo) plugin 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.