Yeah I don't understand this at all. The main reason I don't use Github is because of the high latency. I really dislike their source browser -- it has really low information density and the animation clumsily hides latency.
Chrome is reporting the code tab is taking 3.7 seconds to load. 22 HTTP requests. The Pull tab has over 100 HTTP requests. I don't consider anything over 1s to be fast.
Even the initial HTML is taking more than 1.5 seconds on the Pull tab! I think you should aim for 200ms for the HTML, and maybe 1s total. This is many times slower.
Amazon is pretty decent. Their full page loads are slow, but they concentrate on above-the-fold time which is OK. I don't use Facebook much, but I think they have pretty good latency.
I think Microsoft's new mail is pretty fast but I haven't used it much.
I would say 1.x seconds for initial HTML is average. And 3-4 seconds for the full page with all assets is average. Github is in the 3-4 seconds range for onload time.
So Github is not particularly bad -- it's just average. It's certainly not "fast".
I feel like Github's perceived latency is pretty bad. I think they end up showing spinners and redrawing various parts of page to try to hide latency, but it makes it seem janky to me.