That's also what I experienced. Yosemite UI Performance is super slow compared to Mavericks. Maybe by rewriting the UI Code they also got rid of speed optimizations from the Bertrand Serlet era :/
Turning off transparency in the Accessibility preferences fixed it for me.
And now you get the funny black corners in the hud when changing the volume... Polish is needed.
Funny you mention that. I interned at Upthere last summer and got to know Bertrand a bit. We had some good conversations about the NeXT days. I wish I was still there now so I could talk to him about this. I'm sure he has some interesting input since he knows the software inside and out.