If your Outlook 2007 PST and OST files are large (greater than 1GB) you might want to check this out. Microsoft has released a patch for Outlook 2007 that may offer some help.
According to the KB article:
This update fixes a problem in which a calendar item that is marked as private is opened if it is found by using the Search Desktop feature. The update also fixes performance issues that occur when you work with items in a large .pst file or .ost file.
I’d read the entire KB article before applying the patch as there are some known issues with the patch.
Via Patrick Schmid who says he did notice some performance improvment.
UPDATE: After installing the patch I do notice a significant improvement in performance.