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.