One of my clients travels quite often, and while he's on the road, his internet connection is much slower than when he is at the office. He also happens to have a gigantic IMAP repository, so there is a major slowdown when Outlook synchronizes.
To alleviate this issue, I've setup a client with instructions for setting Outlook to "Work Offline".
For what I'm suggesting my client use this feature for, its not the best name. From what I understand, its essentially the same thing as online mode, but it won't synchronize the entire mail tree upon send and receive, as well as that there are separate settings for how often to check for new email.
I'm hoping this does the trick to prevent the slowdowns. I'll report back what happens!