I’ve got a pretty odd way of internet access for my computer, which I prefer to use due to my Wi-Fi adapter’s slow speeds. It works out pretty well in the end. Curiously, however, Syncthing is the only program that seems to have gotten slower from the USB tether. Now, the tether is not a reliable way to have a connection, of course, but I think having 15 KB/s download speeds is a bit ludicrous. This happened both times I attempted to use it via the tether. I know my upload speed isn’t messing up here as well, speedof.me said my other computer is uploading at 20mbp/s, so it’s not an issue there.
My phone’s OS is LineageOS, I’m using the native tether feature for a little more info. Both computers are running the CLI Syncthing on Windows x64, 0.14.52, same network, just one is running via a tether on the same Wi-Fi.