I think the reason they can do that is because the original has taken the time to develop the characters' personalities so well in the first place, so the AU can "skip to the good stuff".
I agree with this to an extent. but I do wonder...
if we just look at the "latest 50 chapters" of the main manga in the vacuum, isn't it still... a lot more slower in its pacing?
that's the impression I get, anyway.
still, I appreciate all the work that goes there, with focusing heavily on certain characters and smaller moving pieces, but I think that's partially why it feels slow sometimes. like so far in this spinoff EVERYTHING is centered more and more on Iruma. Sure, sometimes we get deep peeks into Asmodeus, Clara, or Ameri, but the scope of focus seem to be in much managable, smaller chunks and repeating across fewer characters.
Like we haven't randomly gotten another Kalego chapter again here, or something like that.
I think the main manga
would do something like that, for better or worse. It's ambitious, to say the least.