Started as quick thoughts, turned into more of an essay, so I've posted the bulk on mediawiki.org:
https://www.mediawiki.org/wiki/User:Brion_VIBBER/ResourceLoader_and_latency
tl;dr summary:
On slow networks, latency in loading large JS and HTML resources means things don't always work right when we first see them.
If we take advantage of HTTP 2 we could skip the concatenation of separate ResourceLoader modules to reduce latency until each module _runs_, without adding _network_ latency.
And if we're more clever about handling 'progressive enhancement' via JS _while_ an HTML page loads, we could reduce the time before large pages become fully interactive.
-- brion