@Ataraxy With a quick local test (using a repository on a ram filesystem) I was able to delete 346 packs almost instantly. I don’t have any numbers for operations against cloud providers though.
But from my experience using some object storage repositories for other purposes (and other services), 0.8/s for a deletion doesn’t seem too bad. I wouldn’t say it’s a great time, but not unreasonable.
To test if this is a B2 or restic thing, you could try using another B2 client and uploading/deleting a few files and see if you get similar performance. If those complete much faster (even for similar file sizes), then it probably be good to raise a bug report with restic. But I have a feeling B2 is the issue here.