It looks like it's 'entities' and not just 'users'. From the docs [0]:

  Keycloak allows you to create any number of realms and any number of clients and users in them. But you need to be thoughtful as you scale up because as the number of entities grows, Keycloak slows down. When you log in as a superuser in the admin panel, even if you have only 1,500 realms, it will take a few minutes or even crash on timeout. Creating a new realm will take about 20 to 30 seconds. You need to change your logic and interaction with Keycloak.

[0]: https://www.redhat.com/architect/oauth-20-authentication-key...