UI Lag OSX Yosemite (10.10) on Retina MacBook Pro late 2013
I'm experiencing some slowdown after a while even on my MBA. It's exactly how you described it. It's perfectly smooth after booting the machine but eventually, after using it for one day or so the animations (especially launching Mission Control, switching between spaces and full screen apps) get noticeably choppy. I don't think there's anything that can be done about it right now, I made a clean install and even executed an Onyx automation, but the problem persists.  This is clearly an optimization problem in Yosemite and I really hope it will get better with the future updates.