In reply to Sarah Gooding.

Of course. :-) I have thought of all that but have settled on one I really like that can't be compressed anymore or it will lose all meaningful detail.

It will be loaded via a CDN, to be sure, but since I had posted this question I looked into it and remembered that in many video blocks (even in Gutenberg's default if I remember correctly) one can upload an image as a "cover" (like many YouTubers do with their videos for instance).

So I extracted the first frame of the video and I intend to use it as a cover. That image will theoretically load instantly - so the visitor will not be confronted by an empty background - and the video will start playing as soon as it is loaded. If there are not enough options to tinker with this in the default Gutenberg video block I understand that there are more video blocks from other developers to try. Do you think that this might work?