I cloned the repo, then was doing occasional `git fetch origin main` to keep main fresh - so far so good. At some point I wanted to `git rebase origin/main` a very outdated branch, and this made git want to fetch all the missing objects, serially one by one, which was taking extremely long compared to `git fetch` on a normal repo.

I did not find a way to to convert the repo back to "normal" full checkout and get all missing objects reasonably fast. The only way I observed happening was git enumerating / checking / fetching missing objects one by one, which in case of 1000s of missing objects takes so long that it becomes impractical.