Same is happening in my 3 node cluster setup. In my case deleting users through the admin console is also very slow, it takes several minutes.
I have moved your question to a separate topic, it is not related to REST API.
I don’t know if your issue is the same as mine. On my end I have identified it as a caching issue. The post below helped.
Adding this system property corrected the issue.
cache.username2roster.size = -1
I hope this helps.