This is a little bit wrong in that the other CPU-bound tasks during loading are generally in the main thread, whereas image decoding is usually on a different t

This is a little bit wrong in that the other CPU-bound tasks during loading are generally in the main thread, whereas image decoding is usually on a different thread (so, depending on how many images you have, and how many CPUs, it may not increase overall page load time in reality). Now, overall time to decode still matters. If it's a gigantic hero image above the fold, then it could take a noticeable amount of time to decode, but it's probably still a win. With the only good JPEG 2000 encoder, its quality at small file sizes is easily better than webp, but in practice it seems virtually nobody uses JPEG 2000, because the open source encoders are not really that great (probably because the patents are a guessing game, although technically the known holders are willing to grant royalty-free licenses).