[nicolasbistolfi]

That's why I added the CDN part, didn't wanted to go to deep on that issue. As you're saying CDN it's a big part of the image delivery, even more important than having a great algorithm to compress those images. We've found that after passing a certain threshold, eg. the image being < 200KB the download time it's not something important, so the size doesn't matter that much, the TTFB (time to first byte) is crucial.

There's also a difference between performance, speed and actual perception of speed, but that is very hard to measure and maybe it's an issue for another topic.