Wow, didn't know a company this recognizable was still using (and embracing!) Rails. Pretty cool. Also helps explain the fact that they've stuck with a mostly server-side-rendered app, despite it being a fairly complex tool.
The perceived performance of Github as I use it as good as any site I use and better than most, so to the extent that's a consideration in client vs server rendered (and it probably should be a big one), it seems to be working.
The new sites with their own implemented loading bars are just super annoying to use. Sometimes they just stall and look stupid. State is bad! Stateless page loads work! Dont use SPAs unless you do something that looks like a desktop app! Please!