I have noticed today that my rMBP running Yosemite was terribly lagging. I mean the interface: Notifications Center, Mission Control and so on. I've gone to Accessibility settings and turned on "Reduce Transparency". Guess what, it did the trick and inteface returned to being buttery-smooth once more. Well, I was a bit upset about that. I have Late 2013 rMBP and it's too new to be already lagging like iPhone 4S on iOS 8. I have done a little internet research and just take a look: http://www.reddit.com/r/apple/comments/2lnytz/yosemite_air_was_crawling_and_lagging_horribly/ It turns out that it's WindowServer process that goes nuts. Killing it and relogging makes interface smooth as it should be. Unfortunately, as soon as you change any setting (even Dark Mode) - it goes nuts once again. So much for quality control. I regret downgrading to Yosemite. I'm too lazy to upgrade to Mavericks now because it's going to take lots of work. Apple is new Microsoft. EDIT: And just as I posted it, I got disconnected from WiFi on 10.10.1. They know what I'm typing..