I've been messing with 10.10 on a two week old Macbook Pro Retina, and in general it's been very very good with one exception. Scrolling. Scrolling in any Apple built app that has differing content has been really jerky. This is expected, this is the first beta. Just went into Accessibility and selected "Reduce Transparency" and immediately the scrolling jerkiness is gone. Literally gone. Not planning on keeping this on forever, but if you are beta testing, and notice this jerkiness, I recommend doing this for the time being. Performance for me in the past hour is now on par with 10.9. Note: The only time I noticed the jerkiness has been in 10.10 optimized app's, and I expect for it to disappear since it wasn't sever in the first place. Just enough for me to notice and slightly cringe. The UI is beautiful, and looking forward for the next build to see if that goes away.