This was my thought as well. Likely the storage used on the WebUI server where the WebUI cache is stored is too slow. It could also be that the storage and performance on the side of the database the root server uses is slow, or other things.
I don’t think the number of endpoints is related to the issue unless the majority of them are talking to the root server directly.
Did these “overloaded” relays get backed up with reports often? 4000 clients on a single relay should not be an issue if the relay is dedicated to being a relay and has fast enough storage / networking / etc. A lower number of clients per relay may help if the relay is getting backed up, but otherwise it will probably not help.