Try using the process of elimination to find what is causing your problem(s).
1) Is it your Identity? Try testing in a new Identity.
2) Is it your User? Try testing in a new User.
3) Re-index your entire drive. Outlook uses Spotlight's index file for search and listing items.
4) Delete the duplicate fonts installed by Office. Probably doesn't cause the slowdowns but will help odd crashes.
Office 2011 Duplicate Fonts to Delete
5) Make sure you have plenty of free space on your drive. 10% minimum required.
6) Delete all com.microsoft plist file, log out/in and let Outlook create new clean files.
Note: Some files have new locations, but old files were still left in original location.
Old location
~/Library/Preferences/Microsoft/Office 2011
New location
~/Library/Application Support/Microsoft/Office/Preferences/Office 2011
7) Normal Template:
a) Track down all instances of pre-2008 Normal template on your computer, and drag them to your desktop. The file is called simply "Normal" and has no extension.
b) Find and drag the file Normal.dotm to your desktop. Unless you have moved it, it should be in
~ /Library/Application Support/Microsoft/Office/User Templates/
Re-start computer after removing files.
Never rebuild an Identity with Exchange. Delete the Exchange and any IMAP accounts before rebuilding. Once you rebuild, it's best to move to your "On My Computer" folder and other data to a new Identity THEN add back Exchange and IMAP data in the new Identity.
How to move your data to a new Identity
Don't let any folder get over 2GB in size.
Identities created by importing from Entourage might have old crap. Even some preferences that existed in Entourage but not Outlook will be imported with no way to change in Outlook. Export and create a new Identity.
Identities created under SP1 in Outlook then upgraded to SP2, could have lingering issues. Export and create a new Identity.
Exchange Basics
The Inbox syncs more frequently than any other and should be kept as clean as possible. (this is the number one cause of problems with sync). Do NOT make subfolders under the Inbox.
If all else fails.....
1) Some users have reported that re-install Office fixed their issues. This is not a fix that I would normally recommend but if all else fails it's worth a shot. See this article on how to remove Office.
Office for Mac 2011: Remove Office
2) If you have the room on internal or external drive, create a 20GB partition. Install Mountain Lion and update. Install Office 2011 and update. Test first without bringing over your Identity. Do you still see the same slowdowns? If no, then bring over your current Identity and test.
Obviously if you have the room make the partition large enough to bring over all your data. Do not use migration. Clone and wipe internal then clone back.