Skins slowing down the CPU is largely a thing of the past... it is more myth then fact on today's current operating systems.
Yes, it is still a problem with older computers running older versions of Windows before Win XP but those machines & operating systems are no longer the mainstream.
Win XP was built from the ground up to run skins; it can still run in the old Classic mode but there is no speed enhancement benefit to do that.
An example is Stardocks WindowBlinds for Win XP. The current version is actually faster then the standard Win XP skins because Stardock uses something called Predictive Skinning which only re-writes the skin every 3rd or 4th scan of the monitor. They found that the human eye can not tell the difference between a writing of the skin every clock cycle or every 4th cycle... it still happens pretty fast.
Today's software has to be designed to perform properly on today's computers. As a result, WindowBlinds 4 only runs all of its features on Win XP. On Win 98, WB 4 is somewhat limited due to Win 98's inherent memory problems.
I am running a two year old 2.2 GHz P4, & Win XP Professional with a LOT of open programs but still am only using about 6% of the CPU time at idle. Yes, I also run WindowBlinds 4 with no speed penalties.