Using Quartz Debug to measure the UI frame rate, they tested the scroll performance of their Facebook news feed on a 13-inch rMBP. Before updating to the nightly build average frame rates were around 20 fps with lows of 17fps. After updating to the nightly build the average frame rate was around 48fps.
Performance is more than doubled! Scrolling is so much smoother. I also ran tests on pages that previously worked fine (e.g. the AnandTech front page) and performance hadn't changed there. I haven't managed to figure out exactly what's changed in the codebase to improve performance so much but it's appreciable.
It's unclear when we might see these improvements in OS X.