I ran a test based on KEYCLOAK-3146, using test module integration-arquillian/tests/other/jpa-performance.

Scenario:
1. Add 500k users via REST API
2. Delete 500k users via REST API

Result:
Adding users took 3 hours on my laptop. During this time heap usage increased to 80-90% (after GC); GC frequency and CPU usage kept increasing. The biggest object was: org.infinispan.container.DefaultDataContainer with ~440 MB. Drilling down I could see ~2 million items in DefaultDataContainer.entries.table[].

After deletion of 150k users the memory usage was over 95 %. GC frequency and CPU utilisation increased as well. Number of items in DefaultDataContainer.entries.table[] doubled to over 4 million, and its size grew to 497 MB.