True, it could be that fast as the Mount.
But even then, the Mount should be slower if it carries a cart over rough terrain, not the cart schould be that fast like a mount.
If you were on streets, it would make sense, that it is faster.
But also in real, a horse is way faster without a wagon behind him.
If the Horse starts to carry wagons faster, they tend to fall to side or break their wheels.
I'm pretty sure that was happen more often in the times when we just had horses that pulled wagons.