Yes agreed it's been a huge problem, and we shipped changes last week to address some of the gnarly p99 interactions. It doesn't fix everything and large PRs have a lot of room to be faster. It's still good to know where some worst performance issues are to see if there's anything particularly problematic or if a future change will help.
FWIW, I find the new React-based diff viewer worse than the old server-rendered page. I disabled the preview for this reason. It does have some nice features but overall it feels more finicky. I would think that in theory this should be better at handling large diffs but I'm not sure that that's the case, and at least the UX feels more choppy.
That's financialization at play. When you render and syntax highlight the diff on the server, Github pays the cost, if you do it on the client side, the cost is paid by the client. At Github's scale it's probably a large enough of a difference that they decided the reduced customer experience is worth it.
