[spiffytech]

You can put a big dent in the impact of the speed of light if you keep round-trips to a minimum.

This is one advantage of server-rendered HTML (though that's not the only option you have).

It also helps that StackOverflow is light on interactivity. You load a page, read for a minute, then maybe click a vote button or open a textarea to discuss. As long as the text and styles load quickly, you won't notice if progressive enhancement scripts take a little more time to load.