You get 250 u/s for free. Everything above that you take one tick at a time, by holding one strafe key and turning the mouse the same way — in the air, over and over. Stage 1 fits inside the free speed. Nothing after it does.
Airborne, hold A and swing the mouse left. Or hold D and swing right. Keep the turn going for the whole jump, then flip on the next one. Land and jump again without ever standing still.
Holding W in the air. Holding a strafe key without turning. Turning without a strafe key. Touching down for even a moment without jumping — that is friction, and it costs you everything.