http://support.microsoft.com/kb/2459115
If the folder-switching performance is still slow after applying the hotfix, it could be caused by one of the reasons below:
Virus Scanner - Particularly applications like Active Scanning.
Add Ins - An add in could be causing it to perform slow.
Complex View - Try resetting back to default view.
Account Configuration Corruption - Remove the IMAP account, restart Outlook, then add the IMAP account again.
