Starting with WoW patch 2.1 we get some nice tools to check addons CPU consumption. Right-click your PerformanceFu in your FuBar and select "Enable CPU profiling". You should then have the top CPU usage for addons when you mouse-over PerformanceFu. You may want to show more addons in the list than the default 3 and sort them by CPU. Note that this add a lot of overhead to the UI so it will increase UI-lag, so make sure you disable it again once you found out which addons are using too much CPU.
EDIT: the (not yet fully documented) API functions for CPU profiling are
here