I'm on a lot of Macintosh discussion lists. It seems like barely a week goes by without me hearing about a user whose Macintosh has suddenly slowed to a crawl. I've helped a number of users troubleshoot the problem, and it turns out that the problem isn't caused by just one thing.

To help users whose Macs have suddenly slowed down, I've created a Web page that lists a number of troubleshooting steps that they can take to address the most likely causes of their slowdown.

Macintosh OS X Slowdown Solutions
http://www.macattorney.com/sd.html