Mostly you shouldn't have to hide backend latency by reordering, and if you don't reorder, you can just use JPEG image strips/sprites, which don't need double compression. JPEGs also stream, but in an order determined by the client, not the server.

In my experience, Dropbox is quite a bit slower (by 10-20x) than services optimized for serving images - using 1500ms to deliver a 25k file is very slow on the web, but it is very common when requesting files via the Dropbox web API. (I can only speculate about the reasons, but Amazon's s3 has big latencies too.)