Not necessarily, the decoding can be stopped once you get enough usable information in regards to the usage of the image (display size ...) with the same source

Not necessarily, the decoding can be stopped once you get enough usable information in regards to the usage of the image (display size ...) with the same source image. That's neat! See the responsive part in https://flif.info/example.html https://flif.info/example.html We can imagine decoding taking into account battery save mode, bandwidth save mode...