For the past few weeks, the download speed on steam via USB-Tethering on my Linux distro (Arch, btw) has been steadily going, barely reaching 2 MB/s right now. I thought it might be the ISP slowing Steam specifically down, since every other download worked at the expected speed (~15-30 MB/s).
Just because I was curious (and wanted to exclude Linux as the issue), I booted into my Windows and started a Steam download, which surprisingly reached 30 MB/s.

When using home wifi, every download has the (approximately) same speed, only with USB-Tethering and only on Linux is Steam much slower than anything else.

I'm at a loss here. No idea what the issue could be.

Update while writing this: I noticed my Wifi was still connected while USB-Tethering was active, disconnected and now Steam downloads with ~10 MB/s. Still slower than Windows but at least some progress and hopefully insight into what could be causing this.