I would say that both thrust and forward speed are important factors for how the plane behaves in flight.
Without enough thrust you never reach the pitch speed or you can't take off from the ground.
Also with a too little thrust the plane lose ability to climb.

The fan's output power is = thrust * forward speed.
The fan's efficiency is = thrust * forward speed / input power

By optimising the ducting one gets better efficiency, which means higher output power for a given power input and fan engine combination.