I have wondered about that, and I'd guess the thing you (and the sibling comments) pointing out about how GH seems to take an html-fragments approach versus GL opting for the more traditional(?) AJAX one probably does matter a great deal on the end user experience

Regrettably, since GH doesn't develop in the open, it's hard to have an apples-to-apples between the two in order to know if GH has some really, really tuned RoR setup, or it literally is just a matter of "don't be cute" when sprinkling every JS trope known to man upon your web ui and if GL one day cared about their page load times, they, too, could be quick without needing a RoR nor RVM upgrade