Performance guidelines

Default user cache settings in Atlassian Data Center products like Confluence Data Center do not automatically scale with the size of the user base so it may be beneficial for reducing the database load to increase the sizes of user related caches.
In Confluence the caches can be adjusted in the Confluence administration -> Cache Management section, then click on Show advanced view

Relevant cachess:
User keys (by username)
Embedded Crowd Users
Confluence User Properties (for cleanup attempt to double the size of the user base)
User Property Set References

Open

You can notice that the capacity is at 100% and they have a very low Effectiveness at default settings. If a user base scan is performed often, those caches have a huge impact on the database performance so we recommend to adjust the size to at least the size of your user base at peak size (+ a little margin, if you are close to the license size maybe set it to license size).
More on Cache Performance Tuning:
https://confluence.atlassian.com/doc/cache-performance-tuning-169119133.html

Collecting the user group membership information for a large user base, like 50.000 users or more can take 30 minutes or more in Confluence so make sure the scheduling does not overlap (choose start time with sufficient interval between).