Has your IMAP connection become slower since upgrading to Outlook 2010? Especially when switching folders? There is currently 1 hotfix available from Microsoft found here:
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.