Comment by JyrkiAlakuijala

During the design process of pik/jpeg xl I experimented on decode speed as a personal experience to have an opinion about this. I tried a special version of chrome that artificially throttled the image decoding. Once the decoding speed gets into the 20 megapixels per second the feeling coming from the additional speed was difficult to notice. I tried 2, 20 and 200 megapixels per second throttlings. This naturally depends on image sizes and uses too.

There was a much more easy to notice impact from progressive images and even sequential images displayed in a streaming manner during the download. As a rule of thumb, sequential top-to-bottom streaming feels 2x faster as a waiting rendering, and progressive feels 2x faster than sequential streaming.