[ken]

That seems like premature optimization, to me. For example, on the meta page linked above, if you scroll all the way to the bottom of the page, it downloads two more JSON documents totaling 55 KB, for all the comments. The page itself is 3.6 MB.

By lazy-loading the rest of the comments, it makes standard browser features like searching and scrolling not work right, yet the bandwidth savings of the initial page load is <2%. You'd have to have a lot of comments on a single page before the cost of simply downloading them all became significant.