Thank you. It would be good to make sure the issue happens with actual WebP served, not with JPEG files sent instead. There might be a format mix-up, too many content negotiation steps etc.
One test you could try is serving a static page with hard-coded WebP images, coming from your CDN and coming from another server. It might help finding the root cause and requesting support to the relevant platform.
One hint I have is that using Chrome Web Developer Tools throttling (right click on your example web page > Inspect > Network tab), I can see images loading
1) Rather late compared to the layout but not as the last items to be refreshed, which seems to be the case in the video,
2) Incrementally (meaning some parts of some images are displayed, then more lines/chunks appear as data comes in) whereas in the video each image "pop up" kind of instantly.
This may be explained by my use of Chrome on Linux compared to Safari on a Mac though, and throttling is not always representative of real-life bandwidth issues.
Estimated quality factor: 84
This image seems appropriately encoded, so unless compression settings change depending on the client environment, we can discard badly encoded WebP files as a potential root cause.