Microsoft have released a patch that fixes a number of issues around the performance of Outlook 2007.
Microsoft indicated that the problem stemmed from RSS feeds, email, and calendar files all being stored in the same .PSD file which as one might imagine could grow in size rather quickly depending on the user.
I loaded the patch and the performance increase is noticeable. Outlook 2007 does have an update facility, but it is unclear if this will be included, as it is not a security fix, also many people have the facility turned off. So i recommend you get the patch here.