Just minddumping here but I think that basically high velocity is bad because it leads to high friction and high temperatures. Velocity has two components, downwards and tangential. Tangential velocity is both good and bad. It increases velocity, but it also makes the trajectory more orbitlike, with less downwards acceleration. A gliding plane will both fall slower both due to the above, but also because thats just how gliding works. But if it tumbles it will not glide.