when an image file is uploaded as a progressive jpeg, this causes it to load extremely slowly, due to the recursive scans involved, and there can be quite a few of those, which constitutes a very inefficient use of server throughput capacity compared to normal jpeg files. whilst progressive jpeg was no doubt an interesting idea in the early 90s, when clients were limited to 2Kbps dial-up modems, there is no longer any sane reason to use that format. and, in fact, whenever I have asked posters why they chose the progressive jpeg format, in 99% of cases they have absolutely no idea why they did it.