I think the confusion is this: in level flight, a heavier plane, all else being equal, is NOT faster than a lighter plane. It's slightly slower since it must fly at a slightly higher angle of attack and create more "induced drag" (drag created by producing lift).

With power planes such as flat bottom wing trainers this shows up as well. When you pick up speed you actually end up trying to fly the airfoil with a strongly negative angle of attack. It's still producing positive lift due to the camber but it's now running with a far higher drag coefficient than when it's flying with a neutral or sligtly positive angle of attack.

Buzzard's points are all dead on