> which with this kind of script reloads the whole HTML,

Trust me when I say that HTML is laughably small in comparison to everything else. If it's beyond 10% on an imageboard, you've probably done something wrong as an adminitrator. If you've set up the site properly, you'll mostly see HEAD or conditional requests, and javascript just isn't going to save you there.

I suppose you could get fancy and implement an incremental style of get, like you mention, but that'll hurt your load pretty bad.