With the power contained in the newest 13″ MacBook Airs l was ready to give up my 27″ iMac for a laptop plus Thunderbolt display. My new laptop arrived today and after a few false starts with Migration Assistant, we got everything moved over and I switched to using my new machine.

While browsing Google Reader during lunch, I was getting hangs just moving between articles. A quick check of iStat Menus showed that two of my four cores were chugging at a constant 70% load, even after I closed all applications. I ran top -o cpu in Terminal to see what processes were burning up the wires and it turned out to be Dock, an Apple OS X internal process. A few minutes Googling lead me to this helpful page on MacRumors and specifically this post. I did indeed have a custom desktop pic on my old machine so this seemed a likely culprit.

I checked the contents of the ~/Library/Preferences/com.apple.desktop.plist and didn’t see anything earth-shattering. It appears to contain only options related to the desktop pictures (if they change over time, where are they located, etc.) Removing the plist is highly unlikely to have any long-term negative consequences, but to be extra careful, you can use these steps that don’t destroy your old plist:

Open Terminal and run

mv ~/Library/Preferences/com.apple.desktop.plist ~/Library/Preferences/com.apple.desktop.plist.bad
killall Dock

This caused the desktop to refresh, loading the default Lion background, and my CPU usage instantly dropped to sane levels. I opened up System Preferences and went to the Desktop pane and was able to quickly restore my preferred background image.