When I upgraded to Outlook 2007, I began to use the built-in Instant Search, and I found it wasn’t that INSTANT. Not only that, Outlook 2007, in general, was just plain SLUGGISH.
The culprit turned out to be my use of large .OST and .PST files. Lucky for me, I was able to find the fix. Additionally, I also was having severe performance issues with the Outlook AutoComplete function. Anytime I typed something into the TO: line or CC: line, my Outlook client froze while looking it up in the AutoComplete. Right when I was ready to uninstall Outlook 2007, I figured out the solution.
Anyway, back to the slow Outlook 2007 performance Fix:
Microsoft posted a fix here: Update for Outlook 2007 (KB933493)
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.
FYI, on a performance note, Microsoft just released a few Windows Vista updates which could improve the performance of Outlook 2007 running on Vista. Take a look here.