I thought this deserved it's own thread.
Whatever they did has completely fixed the scrolling lag for me on image heavy websites.
Lag/scrolling issues simply mean bad optimization of Safari (and other browsers) that handle most load on a single CPU thread.
This is a shame when we have computers with 4 cores and 8 threads, but there was previously no pressure to optimize browser CPU usage since performance at non-Retina resolutions have always been pretty good.
Apple saw the need to improve Safari's performance for Retina displays, and that's why they though about using CoreAnimation for GPU-accelerated scrolling in Safari 6. They may have though that it would have been enough to avoid lag, but it was not.
A single CPU thread still gets most of the load and that's where the bottleneck seems to be. Apple is currently working on a new version of Safari with much better optimization. You can already get an early taste of it by downloading WebKit's nightly build. It's an in-progress, open-source version of Safari's next version.
WebKit fixed the lag perfectly for me, and it's just a matter of time before it gets a stable release which will probably be Safari 6.1. It's unclear whether Apple has improved CPU threading or GPU acceleration with it (or both), but it works.
Whatever they did has completely fixed the scrolling lag for me on image heavy websites.