In case it is the Repository, the mentioned article could help but there are other possible causes. For example it could be your User Directory Synchronization causing this if you are syncing a lot of users. It's a good idea to look at the Repository -> Trace -> System.log to see if you can find recurring error messages in there.
Hope this helps!
Bastien Laugiero If a post helps to resolve your issue, please mark the appropriate replies as CORRECT.