Land Rovers were not designed to tow at high speed. They were designed to pull very heavy loads at moderate speeds. Think of them like a tractor, a tractor could never be described as fast, but no one could ever consider them to be crap at towing. The Discovery's weight and aerodynamic qualities will always be a limiting factor as to how fast they can go. Sure, a reasonably powerful car may tow a 1000-1500kg trailer as fast as, if not faster than a Discovery, but when you take that to over 2000kg it will really start to struggle, unlike the Disco, which will pull 3500kg just as easily as it will 1500kg.