One of our clients reported that Snow Leopard's iCal is slow to bring up matching invitees for a meeting, especially if you have thousands of Address Book Contacts. We did some research and this is what we found.

While it looks like Apple may need to address this in an update (10.6.3 is marginally faster than 10.6.2), but until then, here's a quick tip on how to deal with a slow iCal when you have a large address book:

1) Create the appointment, but don't click on the "Add Invitees..." like you normally would
2) Instead press Option-Cmd-A (or select "Address Panel") in the Window menu
3) Search for the contact in that window and drag and drop the name into the "Add Invitees..." area on the appointment

It's not super intuitive, but it's a lot faster than waiting for that spinning wheel to disappear.