restic: restic prune is slow

Output of restic version

restic 0.7.3 compiled with go1.8.1 on linux/amd64

(actually running github master)

How did you run restic exactly?

restic prune -o b2.connections=40 --repo b2🪣/path

What backend/server/service did you use?

backblaze b2

Expected behavior

complete the task in a reasonable time

Actual behavior

Prune is very slow, at this speed it will take 7 days to complete.

will delete 2440 packs and rewrite 19141 packs, this frees 31.369 GiB [30:42] 0.29% 56 / 19141 packs rewritten

Do you have any idea what may have caused this?

It looks like prune do not use multiple connections, lsof shows only two connections to B2 despite the parameter b2.connections=40