An interesting read.
I've just started with Hetzner, and their storage box product to try cloud backups via Restic.
Whilst very competitively priced, the 3MB/s transfer rates I am currently achieving on a 1000/400 fibre connection means I am unlikely to ever use the 10TB I have supposedly been allotted.
I also use the storage box and easily have 30Mo/s bandwidth (and 130Mo/s+ from a hertzner server), so that's strange that you seemed capped at 3Mo/s. Are you not in Europe ? You should probably contact the support to report it, because that almost definitely now how they intend their product to be.
I had a similar issue with an older version of Restic + Hetzner, update to latest stable, if you haven’t yet. It improved my speed literally 10x, something to do with buffering over ssh.
Hetzner servers aren’t billed against transfer quotas to storage box, and get a direct, faster link.
It sounds like you’re connecting through the public interface, for which many ISPs in the US rate limit the Hetzner storage boxes. In my case, around 3mbps.
Using a VPN from your home (unintuitively) increases your throughput, alternatively use concurrent webdav (HTTPS, typically not rate limited) without VPN.
Said plainly, the symptom you’re seeing is the same reason I could likely guess your ISP, and has nothing to do with Hetzner.