I recently upgraded a client to Office 2010.
Amongst the many teething troubles were a couple of users complaining that going in to their eMail was now very slow. The problem looked like this:
Well, the fix was actually very simple: Outlook 2010, unlike previous versions, scans for new mail on startup in every account and pst.
If you have a missing Archive PST (Maybe you archive to a PST on an external drive which isn't currently connected) or you have been granted access to another user's eMail, and they have since left and been removed from Exchange, Outlook 2010 will hang for a stupidly long time before timing out. But without doing anything sensible, such as showing you an error message. You've got to love Outlook, eh?
The fix turned out to be fairly simple: Remove the other user's mailbox from this Outlook. (Or remove the link to the Archive or make the archive.pst available, if this is the problem)