A simple search on Google on Outlook Performance Issues will reveal the story. Microsoft have release an update that

  1. Fixes performance issues that occur when you work with items in a large .pst file or .ost file
  2. 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.

In addition to this Microsoft update, Amit Agarwal of Digital Inspiration has some suggestions and fixes for you.

What has been your experience with Outlook 2007 so far? Have you gone back to Outlook 2003?

I use Outlook 2007 at work and have not noticed anything unusal.  However, I do have some tips lined for Vista coming up next week. Stay tuned.