But it can keep the port open so a few round trips are saved, it might be fast enough to make the second request while the first was still coming in so effectively very little overhead, even on (especially on) high latency links where it matters most. It can start drawing even from the first range request, and the idea is to never pull down the entire file (for big files, the case you are optimizing anyway, small ones you don't really need to) and only maybe end-up pulling in the entire hires file with a third request if say the user zooms way in.