Switching back to 2003
Jason Clarke, on his blog, reports:

Clearly I'm not the only person in the world that has experienced slowness in Microsoft Outlook; "Outlook slow" is the second most searched-for phrase that leads people to this site, behind "eye twitch".

Scrolling down to read comments, one of the correspondents on Jason's blog writes:

Outlook 2007 literally hangs my entire system at the end of checking POP accounts. Incredibly annoying and seemingly being ignored to this point. I'm considering switching back to 2003 soon if I can't find a fix.

Outlook 2007 much slower than its predecessor
Amit Agarwal, on his Digital Inspiration blog, reports:

Microsoft Outlook 2007 is painfully slow. Whether you are running a trial copy of Outlook or a licensed version, the performance of Outlook 2007 software is much slower than even its predecessor, Outlook 2003.

The software freezes during start-up, the status bar shows "Loading Dataset.." for minutes and you can perform no actions on your email items unless that initial "Send/Receive" is complete.

What to do about it
If you've made the move to Outlook 2007, Amit has some good suggestions. He recommends:

• Disable RSS Feeds in Microsoft Outlook 2007
• Disable Outlook add-ins you no longer use
• Reduce the size of your Outlook.pst personal folder.
• Start Outlook 2007 in Safe Mode

The cool folks over at Outlook add-in maker ClearContext have a helpful set of recommendations. They have a video which shows how to reduce the size of your mail file and use ScanPST to check for errors. It's a worthwhile video to watch.

Microsoft also has a Knowledge Base entry on the topic. They recommend:

• Reduce the size of the Exchange mailbox
• Use an online mode profile instead of a cached mode profile
• Split your large .pst file into several smaller .pst files
• Use a filter to restrict the items that you synchronized from Exchange into your .ost file

What we're doing about it
It's time for us to fess up. Here at OutlookPower, we are not upgrading our personal desktop machines to Outlook 2007, at least for a while. Yes, there are some slick new features in Outlook 2007 (we're particularly excited about the upgrade to Business Contact Manager), but we simply can't afford the hassle.

Personally, I get more than 7,000 email messages a day. That's after two levels of email filtering at the server -- before it even reaches my PC. I spend way too much time in email already, and there's no way I can justify spending more time fighting with freezes and stalls. I don't have the time to futz with my .PST file to reduce its size. What I have works fine in Outlook 2003 and it's going to stay that way.