The February Cumulative Update for Outlook 2007 has been released – it contains a significant number of new performance-focused improvements.
The Outlook team say they especially focused on the areas of starting up and shutting down Outlook, switching between folders with large numbers of messages, calendar reliability and consistency, and general responsiveness of the application when performing common tasks around managing your e-mail, calendar, and other information. In short, they believe your daily experience with Outlook will be greatly improved with the installation of this Cumulative Update.
You can find more info here: http://blogs.msdn.com/outlook/archive/2009/02/25/announcing-the-february-cumulative-update-for-outlook-2007.aspx
The download is here: http://support.microsoft.com/?kbid=961752
A whitepaper here: http://support.microsoft.com/?kbid=968009