If you check the Network tab of the Browser Web Inspector (in Chrome View -> Developer -> Developer Tools), the files that Pagespeed refer to for Youtube are not being loaded, until the placeholder is clicked.

To prevent the iframe from loading anything, Responsify replace the iframe src attribute with a data-src attribute before Joomla renders the content. Without a src attribute, the iframe content won't load.

You can see this in the source code of the site:

<iframe referrerpolicy="strict-origin-when-cross-origin" title="Berry : production de citrouilles géantes !" width="710" height="400" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" frameborder="0" allowfullscreen="allowfullscreen" class="wf-responsive wf-responsive-full" data-src="https://www.youtube.com/embed/NBuujvMVdCI?feature=oembed&amp;controls=0&amp;rel=0&amp;modestbranding=1&amp;privacy=1" loading="lazy" ></iframe>

When the placeholder is clicked, the src attribute is added back with the value from the data-src attribute, and the iframe loads.

It appears there is some other script - not Responsify - is adding the src attribute back when the page loads. Do you have some kind of "lazy load" script or process active on the site?

I may have managed to find the rights settings, with a balance of loading performances enhancement and Responsify correctly working. Unfortunately, I didn't manage to understand what was the cause of the initial malfunction, so I don't have an interesting feedback to give here.

Great, as long as it is working! 😀