Thanks Jamal, over the weekend I re-designed my entire website from using Blocks back to classic editors.
Not only did the waiting issues disappear but I finally was able to remove all the junk scripts added for Gutenberg (mainly by WP Itself), back to a lean and fast WP.
As for the issue itself, I still see it on practically all sites I admin, of course, all those sites have a tad more than 10 posts.
But they are all on excellent servers and host systems, so I don't really think it is the host or server, also because I am able to replicate this easily on a local MAMP by importing those sites.
The cause of the issue is simple, I think, at least according what I observe.
The background processes of the new editor which constantly updates the content (at least at WP Heartbeat rate, and at each edit), specially in Views but not only, when you are editing, tries to constantly update the preview.
With one or two posts that works well. With 10 or 20 already it can already hiccup a bit, but imagine more, with complex Views and loop designs.
What happens - as said observed on practically all sites I "have" - is that first the black messages I show in the first post appears.
Then, the blue WP button starts partying and says it is updating, then it "greys out", or becomes inactive in this sense.
If yo are lucky after a while it comes back active. If not, you just lost all edits and need to reload the page, praying that it saved it anyway, but more often than not, it will then result in those famous "corrupted block" or such messages.
Anyway, as mentioned I bit into the sour apple and have redesigned the website using HTML, CSS and a bit of JS.
It works (duh, pun intended) and I don't have a copy of it anymore in the previous state.
I do have another site which does not belong to clients (which are all too heavily customised to even talk about support), but that site has 80k posts and I assume I know the answer to that 😉
So I am closing it here, it was fund sharing the Gif though.
I hope one day you can catch this sketchy memory issue or whatever the like it is :heart: