This. I’ve had issues at work while imaging classroom computers where some would finish in ~30 minutes and a few would need hours. All of the computers used Cat6 cables. This being a classroom, and students being absolute wankbags, they kept yanking the computers and kicking the cables, so the wires came loose from the plugs. I later used ethtool to debug the slow computers – the switch would only allow 10baseT link modes.