THis is usually due to the user having hundreds of “subscribed” or watched folders. Usually they're not aware of this at all.

Fix (from an email I sent):

Forgive the haste with which I type these guidelines. I'll try and make them more detailed and easier to follow in time. Here are the things I do when someone tells me that their outlook is hanging, or taking forever to get started when they work with the IMAP server:

     1.  Tools->Options; Mail Setup tab; click "email accounts."
         Select your CS IMAP account.
          -> More settings -> Advanced
          Set "Root Folder path" to be the directory where your mail
          folders are stored.  Using a ~ tilde is OK, i.e.,
          ~/work/mail or ~/Mail .

     2.  From the main outlook screen, left pane, right click on "CS IMAP"
         or whatever you've called your CS IMAP account connection.
         Select "IMAP Folders", click "query" and watch as a list of
         folders is built... every file underneath the "Root folder
         path" you selected in #1, above.  Subscribe to every flder
         that you want to see at the left of your outlook screen under
         "CS IMAP".

     3.  Tools->Options; Mail setup tab; Send/Receive.  Click "edit";
         select "Use the custom behavior below."  Pick and/or unpick
         folders that will be checked for email EVERY TIME you run
         send/receive.  The fewer folders there are in this list of
         watched folders, the faster your email checkups will be.
         Also while you're in this area, unclick "get folder unread
         count for subscribed folders."

Good luck,

   MCV>