That doesn't actually speed anything up; it's just an illusion that, depending on the user, seems to either make rendering seem faster or slower (K-Meleon, the fastest Mozilla-based browser for Windows does the opposite). What
does actually speed Mozilla up on new servers is Pipelining, which is basically an experimental feature that takes more info in each packet of data from the server, thereby reducing the amount of time taken to load a page. There is a UI preference for it in Advanced/HTTP Networking.
