Even with autosquash, rebasing has to iterate through each commit individually before creating the final commit. Which means it's touching a bunch of files in your working tree while that's happening, which means build tools like make now have to rebuild them because the timestamps have changed.
What's worse, is if you rebase (say, 20 commits) onto a newer branch than the one you started with, and you have merge conflicts (say, on commit #10), now you might have to fix the conflicts over and over for every commit after the first conflicting one (depending on the contents of the commits). I know there's `git rerere`, but after 16 years using git I've still never learned how it works.
Using reset to squash first, guarantees that you create your single commit first, without dealing with merge conflicts/etc, and then you can rebase that resulting commit once, to deal with conflicts once.
