On April 13, Microsoft released an update for Microsoft Office Outlook 2007 that addresses performance problems with large mailboxes. The Microsoft article "Description of the update for Outlook 2007: April 13, 2007" describes what the patch fixes. I regularly use three different Outlook 2007 profiles against three different Exchange Server organizations, and I've occasionally noticed poor performance on one particular server. I eagerly installed the patch and found that, as promised, it helped improve the performance of Outlook 2007 on that server. However, I decided to experiment a bit and found an even more dramatic improvement from doing something that I'd never done before: I turned off Cached Exchange Mode.
I decided to turn off cached mode after receiving an email message from a friend who was extolling the virtues of using the online mode; among the benefits he mentioned was the nice break from email when he was flying or in other environments where he couldn't get a live network connection! I have a desktop computer that enjoys good connectivity but has limited disk space, so I thought I'd give online mode a try and see how my experience went.
As with most other Outlook users, I welcomed with open arms the advent of Cached Exchange Mode in Office Outlook 2003 because it smoothed over bumps in network or server performance, along with giving me a reliable offline copy of my mailbox data. Turning cached mode off to get better performance seems a little counterintuitive, especially since the reported problems with Outlook 2007 all had to do with performance on the client and not with cached mode itself. The problem was caused by changes the Outlook team made in Outlook 2007 to better support Windows Desktop Search (for more details, see Will Kennedy's blog post. When working with large .ost or .pst files, Outlook 2007's performance could be significantly slowed, sometimes in unpredictable ways. The recent update improves Outlook performance with these large files, and in my testing (and in reports from other users and Microsoft MVPs), it seems to do a great job. I commend Microsoft for moving quickly to address this problem instead of making users wait for a full service pack release.
What about my testing? My experience with online mode has been good so far because all three of the servers I use with this machine are lightly loaded, well-maintained, and well-connected to the network. I plan to keep cached mode active on my laptops because when I travel I often need offline access to email—to say nothing of the horrors of finding connections in random locations around the world! Cached mode is also required if you want to use Outlook's Junk E-mail Filter; it doesn't run when Outlook is in online mode. Furthermore, cached mode offers the advantage that your email delivery appears to continue without interruption unless the server is completely unavailable. Don't underestimate the importance of this last advantage; many users (and administrators, and help desk personnel) have found it to be the best reason to stick with cached mode.