Ever since upgrading to Yosemite, after a few hours of using my computer, the interface would start to lag more and more, and over the course of a day it would become so slow I'd get really annoyed. After a restart it would be fine again and then it would start lagging eventually. This only affected the animations such as switching Spaces, Mission Control, Scrolling, etc... System performance was unaffected. Then I found this article: https://david.gyttja.com/2013/01/21/fix-lagging-display-performance-on-retina-macbook-pro/ Here's what it says: Delete /Library/Preferences/com.apple.windowserver.plist Delete ~/Library/Preferences/ByHost/com.apple.windowserver*.plist Shutdown OSX Startup, immediately press and hold the P and R keys while holding down the option (⌥) and command (⌘) keys before the gray boot screen appears, which resets the PRAM You may have to reset your display preferences (resolution) once you login Done! So far my computer has been running for a few days without any interface lag. True, I have not used it a lot, and it tends to lag after many hours of using Photoshop, which I haven't done much since then. But even like that, it feels a lot faster than it used to be. For those having lag, this is the thing to try!