We use Contentful CDN to optimise images dynamically based on viewport and browser. One of the enhancements we have is WebP if the browser supports it. The enhancements are added using parameters at the end of the image URL, which is usually a JPG.
It appears that the Webpagetest waterfall view only displays information for the jpg images, even if a test browser is selected which supports webp. The image asset used by Webpagetest is the fallback (jpeg) image and it hasn’t been been resized. An example test is here:
The optimised URL for the main product image I can see in Chrome on this page is: https://images.ctfassets.net/bz0fvtkbk5r1/1RTV7epqaHfVe9hr1xOCfg/dd427dae55c6d6f208065f32da2bb865/PD859BCR-main.jpg?q=80&w=550&fm=webp
and the one shown in the waterfall view is: https://images.ctfassets.net/bz0fvtkbk5r1/1RTV7epqaHfVe9hr1xOCfg/dd427dae55c6d6f208065f32da2bb865/PD859BCR-main.jpg?q=80&w=1750
Thanks in advance for any help & advice.