It's not unuseable, but it definitely feels like 'js hacking my scrolling' and not a native surface flinging around.
The experience is actually worse with smaller movements, i guess because my brain is more conscious when breaking the 'finger physically moving the text' illusion.
I don't mean to be dismissive - you're working on a really hard problem, and you're clearly approaching it with a mindset of perfection. I'm posting because I know you're probably able to solve this too :)
Edit: as a point of (unfair) comparison, the codemirror Huge File demo works fine: https://codemirror.net/examples/million/ It does suffer from the occasional partial paint when quickly coasting, but I'm not bothered by this at all, it's far less intrusive than dropping frames / stuttering / etc.
