I don't see how GH's backend serving a billion repos would affect the speed of their frontend javascript. React is well known to be slow, but if you need numbers, you can look at the js-framework-benchmark and see how many React results are orange and red.
Sure, React has overhead. No one disputes that. But pointing to a few red squares on a synthetic benchmark doesn’t explain the actual user experience on GitHub today. Their entire stack has evolved, and any number of architectural choices along the way could impact perceived performance.
Used properly, React’s overhead isn’t significant enough on its own to cause noticeable latency.