> The trouble is that images are a large fraction of the page size even for well optimised websites. Using smaller images via changing format often makes a significant difference in the page load time.

Pepperidge farm remembers Google heavily pushing for WebP for this reason. Look how much smaller it is! and Lighthouse giving demerits for every non-WebP image. Which, of course, is totally true, WebP images were almost always quite a bit smaller. They all looked like shit, too, of course.