Personally, my Firefox Nightly on Android will regularly crash (along with the whole System UI) when opening / trying to type into a GitHub page
There were a couple GH discussion threads about the dreadful UI rollout, but someone complained one too many times about (code) search returning the wrong results so they've since been locked. For me it's the issue search that seems to consistently return the wrong results, and the text input widgets that are glitchy as hell in Firefox.
Performance-wise it's worth keeping in mind that Github is now rendering all the text client side. If you go through the discussion threads you can see where the GH devs didn't know how to account for multibyte codepoints and how that broke their non-native rendering with things like emoji and Chinese characters.
Stale data, PR’s and such don’t update properly, page loads are glacial, half the time it’s doing some loading bar of its own-no doubt as part of some trying-to-be-clever SPA thing, and it’s awful.
There's a hanging draft for it in Gitea (inb4 yes also Forgejo).
https://github.com/go-gitea/gitea/pull/20311
Maybe this is the push I need to actually start using taskwarrior...