Can you turn on developer support info and database statistics and login to the sytem to view "my space" of a user that is not admin and send us the request information for that request. maybe we can find out what the bottle neck is from that info.
Also shut down webct for a short test to see if makes a significant difference.
Can you also post your config.tcl without any sensitive data?
Also, can you post your authentication, kernel and main site parameter settings under /acs-admin/
How many authorities exists for your installation? Does it make a difference if you deactivate your URZ Heidelberg or Extern authority?
Have you ever tried postgresql instead. My installation with 22.000 users is quicker.
