Yeah, I know, my point is that ordinary installer should do the same with ordinary installer UI rather than trying to install every deb package over and over again, running its pre/post scripts, etc. What's the point? It should be deterministic anyway.
As for Debian, the slowness comes mostly for fsync calls I think. You should be able to use eatmydata to avoid this. See: https://people.skolelinux.org/pere/blog/Quicker_Debian_insta...