Is the feature really saving that significant of load time? I mean HN is as minimal as it gets, I'd rather all the comments load and take 3-4 seconds more than have to manually go through and hit more several times.
As pointed out by pg above, it seems not to. An educated guess (read: wild speculation on my part) indicates that, due to the ranking algorithm of HN (versus Twitter's simple sort, where new things show up first and order never changes) rendering and bandwith time are probably dwarfed by the time required to load and sort all of the comments on all pages to determine what comments show up on the first page.