> What they should do is give a popup saying "You have a slow cable, it could be faster, go rummage till this message goes away and mark the good one with a sharpie", and that wouldn't need any changes to the spec

I think something like this exists in Windows. I'm not sure under what conditions it triggers, but I remember occasionally seeing a tooltip saying something like "this device could work faster", directing me to plug it into a different USB port.