My personal opinion about this is : W does not reduce your speed while you're in air.

In fact, while in air, you gain speed by strafing with keyboard and converting your strafe speed using your mouse. But you can only convert strafing speed, not forward speed.

If you look forward and only press "D", your strafe will be tangent and you will increase your speed efficiently when rotating to the right. If you press D and W, some of the strafe vector potential will be lost, because it will push forward, where your speed is over the maximum speed limit already.

Imagine you're against a wall : you will move slower pressing D and W than just pressing D. Same thing if you press W while midair, all your lateral movements will be diagonals, and the "forward" speed is ignored.

One more thing : you have to use wasd key relatively to your movement. If you do some lateral jump, where you move to the left and look forward, you can use W to strafe right, S to strafe left, and A become the "useless" key.

tldr : imo, you will not go slower by pressing W, you will accelerate less.