The update for Outlook 2007: April 13, 2007
Outlook 2007 can be pretty slow if your PST files are large. Apparently Microsoft is aware of this issue and the company released an update that should help to rectify this issue:
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.