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.
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.
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.