I just got a new Mac mini, and noticed it was crawling along. So I used Activity Monitor, and discovered that the mdworker and mds processes (both related to Spotlight) were constantly using 60% of CPU on both cores. After a bit of digging, I figured out that this was due to the constant indexing of the destination folders of some torrents I was downloading.
The fix was creating a dedicated folder for my torrent downloads, and then adding that folder to the Privacy tab in the Spotlight System Preferences panel, so Spotlight no longer indexes that folder.
Now the mdworker and mds processes are behaving normally and my system is running much faster.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090328012408905