Here are my findings on my Retina MacBook Pro with ML GM.
-With discrete graphics and native resolution, everything is butter smooth.
-With discrete graphics and scaled, OS animations are smooth and some sites may slightly stutter in Safari. In Firefox beta, however, everything is butter smooth. So, Apple has some catching up to do. It really looks bad that they are beaten by firefox in this department. On the iPad, the situation is reversed.
-With both integrated and discrete, Firefox scrolling is MUCH smoother on web sites Safari has problems with (such as Google+, Facebook)
-With scaling via integrated graphics, animations and transitions are sometimes smooth and sometimes stutter depending on what you have going on.
I have no doubt that scroll lag will be addressed. What concerns me more is how well they can address interface animation stutter when running scaled via integrated graphics.
Also, I have a 17" MBP early 2011. Honestly, scrolling isn't really all that smooth with integrated graphics either. In other words, the focus being placed on this topic is magnifying the importance of butter smooth scrolling on large web pages.