> Everything you do needs to be synchronous and almost everything requires a reload.

this is pretty incorrect, you may want to look into the concept of "partials" in SSR. maybe you meant everything requires a roundtrip ? but SPA would not solve most of the roundtrips necessary in github given many interactions in the github app require authn/authz checks.

would you care getting into more details ?

Also, 'old' github was known to be very fast an reliable and was indeed a ruby on rails SSR app. Since a few years ago github started to introduce react and more client side logic and it correlates with more issues and more slowness in the frontend. It only correlates, but still.