Thought I'd come in and clarify a few things that have been said in this thread and others. There is indeed much less lag and things generally perform great but retina macs are still struggling with transparency in some cases (try opening the reader sidebar in safari when in the top sites page and look at the frames, or try simply swiping between spaces with this screen open). Also there is still lag in plenty of areas including iTunes scrolling, grid stacks in the dock and most annoyingly, PDF scrolling. The OS X pdf scrolling performance continues to be atrocious, lagging behind what my cheap android phone can do with the same files. Something really needs to be optimised there... Apart from that, the performance upgrades are very good.