I know that I'm simplifying (probably too much), but it seems like things were fine when GitHub was still a Ruby on Rails monolith and all the rigmarole with microservices etc. only made things worse.
This sounds more like a belief, based on little more than "correlation is causation", than analysis that controls for macro-trends backed by evidence.
Unless everything else stays the same (underlying traffic etc.) then you can't really compare. Could be that you hit some fundamental scaling limit with the old design and it completely falls over after a certain scale.