We have been a long time user of RDM and what we have seen is that the client refresh has slowed down our client performance we are currently at 5500 sessions and every time a refresh is executed it freezes the client for about 10-30 seconds.
In our old RDM 8.2 on MSSQL we set the auto refresh to every min on the clients and this worked against us in the past with RDM performance really dragging as more users started to use RDM.
After we went to the new RDM server 3.2.1 we no longer do the auto refresh but we see it does refresh before every edit. The problems we see is edits are only seen by the editor unless all users exercise a refresh. So we would like to implement the auto refresh every min but as a suggestion i wish that the server and clients always retained an internal server update version and when a refresh is requested instead of forcing a full refresh of the list it would first compare internal update versions and if the same then no refresh is executed. If the client is below the server version then a forced full refresh is required. This way only clients that are behind on updates are refreshed and clients that are same version the refresh is skipped. I feel this would make the whole RDM experience better for the whole team.
Also maybe if a refresh only updated the session changes and not the whole list this would be an added bonus and truely speed things up.
Does your caching mode is set to Intelligent in your data source configuration?
Could you post a print screen of the Advanced tab of your data source configuration in File -> Data Sources.