🙄The link I refer to is the response - a table outlining the conditions when and how Shopify will compress images published by Shopify. How's that vague? It just states they don't necessarily recompress every image which you initially stated they do.
Yeah they claim on average webp can save up to 25% vs jpeg, but not sure if that holds up in practice, somewhat depends on the image. And if you're really interested in optimizing page speed, then I'm not sure making images 15-25% smaller is really going to do it. You'd be much better of lazy/late/on-scroll loading images below the fold rather than loading an entire page full of slightly smaller files.
I hear there is another new image format in the works, which will be open source and supported by all browsers, based on a video compression method that gets way better compression rates than webp, but it may not be available for another year or so.
Shopify now supports WebP. Not sure if it's just Plus though. From the Plus update team this weak: