I opened rdar://17124873 for this issue. 2-3 windows are enough to make the entire thing become so sluggish, similarly terrible as iOS7 on iPad 3. What an amazing difference between this and Mavericks beta 1 and its smooth scrolling. Open 2-3 tabs in Safari and try to scroll - an exercise of patience. It is either a terrible driver for the Intel 4000 chip, a terrible optimization or the chip cannot handle the blur. Since I've run Windows 7 on this machine with all the Aero effects running smooth at full retina resolution and another display, I will take a wild guess and assume the former two.