This is an enhancement in WebKit, not Safari. ≈ 100 percent more performance (2x)....
B) *this one likely only affects Retina users* There has clearly been an effort to increase the fluidity of the OS (animations, etc.) yet I notice mixed results on my mid-2012 2.3ghz retina MBP. Safari scrolling is better but still tends to lag a bit on pages like Facebook.
Use the QuartzDebug Frame Meter. It should show you the current framerate of the GPU.The Dock magnification lag has been addressed, and and to my untrained eye it appears they may have implemented frame-skipping with the intel HD integrated card.