Microsoft on Friday released a patch for Outlook 2007 that addressed a number of performance issues (described in KB933493). Complaints of sluggishness have been circulating since before the product was formally released, typically where large OST (offline folders used for synchronization) and PST (personal folders typically used for archiving) files are involved.

From Computerworld:

The 8.3 MB update should accelerate the download of messages from the Exchange e-mail server and reduce temporary freezes resulting from deleting messages or copying them from one folder to another, according to Jessica Arnold, Outlook’s program manager.

The update should also let Outlook 2007 users switch between messages faster and enable faster program startup, she said.

I've installed it and will see how things go. Right off the bat Outlook started up faster, so that's encouraging.