Very strange bc it is not the first time I notice this issue.
Now I was trying in Firefox for a change, and there it was immediate.
Then I went back to Chrome and it went fast.
So maybe the context is important : I was busy doing all the edition work when I faced the issue.
So maybe something happens then, something looping or whatever... which is solved once you quit and re-enter the newsletter.
I'll post again if I can assess myself what the more precise context is. But it could be good that @acyba takes a look into it anyway 🙂