So it has something to do with video and/or QGC. I started with a fresh BlueOS 1.3.1 installation this morning. Same results as before when QGC was running. Shut QGC down and voila, back to normal:
I’ve used tether setups with as little as ~15mbps bandwidth with video and a high-def sonar with no problems, so something is clearly weird here for the video stream coming from the vehicle to be crippling throughput with more than 70mbps capacity. Interestingly, when I connect to the vehicle with Cockpit instead of QGC (after multiple reboots of the vehicle) I get no video stream (and no decrease in speed/bandwidth). I haven’t changed any settings in Cockpit and it did work early on in my setup attempts with this OTPS system.