I think it's just because it's a horizontal animation that's activated by a vertical scroll event, but if I scroll quickly it looks pretty janky (in Chrome 64 on a relatively new iMac).
Interesting. I'm not sure if I can do anything to improve the performance further. The frame rate stays by almost 60fps and the amount of repaints on the site has been reduced to a minimum. It runs smooth on my older retina MacBook, but I can also see a subtle janky animation when I scroll fast.