I hate to be a negative voice but that's the first thing I noticed too. Using Chrome (desktop) and the site just feels all wrong (by re-implementing scrolling, I guess?)
Scrolling seems like one of those things that shouldn't be touched by a 3rd-party library and should be handled by the browser/OS and only the browser/OS (in most cases -- I'm sure there are times when it's necessary, but this doesn't seem like one of them)
it's jerky on my (haswell) desktop too. It seems that one of the things they've done to make it feel faster is to mess with the scroll wheel - it scrolls slightly more than the default which makes using the site feel sort of "fake".
It seems like they heavily turned on scroll acceleration. If I scroll 1 "click" of my mousewheel the scroll bar only move a tiny almost imperceptible amount.
If I scroll a lot the bar will fly off the top / bottom of the scrollable area and slowly come back.
Jerky animation is not their only problem. The layout is incorrect with a lot of letters clipped off. The mobile menu breaks when you tap on it a little too fast.
