Normally, Cairo-Clock with the smooth needle movement, it's at least 10% for the Clock, and 12% for X. That's quite heavy for a mere clock ! Hopefully, Cairo-Dock2 solves the problem, and now has a Clock applet which is compatible with the Cairo-Clock themes, has the smooth movement, and takes ... 0% of CPU ! yes, almost nothing, making your CPU get bored and your battery last longer. Here is a short demo, not very readble though. the CPU load is due to gtk-recordmydesktop. http://www.youtube.com/watch?v=QuanKtZEnSk
How to : install Cairo-Dock2, activate the Clock applet, drag it out of the dock on your desktop, you're done ! you can resize it with alt+middle-click, and rotate it in 3D as much as you want ^_^