In your code you apply a one-time force (impulse) to the ball. The speed is just a result from applying the impulse. So you named your variable wrong. How fast the ball will go will also depends on the set mass of the rigid body.
To slow down the ball while it is in the air I would add a "draft" force/impulse in opposite direction of the balls velocity. Probably you will also have a floor so the friction will bring the ball to a stop.
A rough estimation of air draft will be that it is proportional to the square of its speed. So in this code below i put the square of the velocity (length of velocity vector2) in sqrVelocity. Also the applied impulse is multiplied by delta so the draft_factor basically stays for the draft force applied in one second. (depending on velocity)
export var draft_factor = 10.0
sqrVelocity = linear_velocity().length()*linear_velocity().length()
apply_impulse(Vector2(),linear_velocity().normalized() * sqrVelocity * -1 * delta * draft_factor)
You will have to try out the draft factor.