Racket-head speed is built from the ground up. The arm is the last link, not the first.
New players try to smash with the arm. Experienced players smash with the floor. The difference is a kinetic chain that starts at the feet and ends, almost as an afterthought, at the wrist.
It begins with a rotation. As you load onto the back foot, the hips coil away from the net, then drive back through as the front foot plants. That rotation is the largest source of racket-head speed you have, and it happens before the arm does anything.
Power is a sequence, not a muscle. Break the chain anywhere and the head slows before it reaches the shuttle.
The shoulder and elbow relay the energy upward. Keep the arm relaxed until the last moment — a tense arm cannot accelerate. The forearm pronates and the wrist snaps only at contact, adding the final increment of speed rather than supplying the bulk of it.
Contact point matters as much as speed. Meet the shuttle high and slightly in front so the face angles down into the court. A powerful swing that connects late sends a flat smash long every time.
Drill the sequence slowly before you add force. Shadow the rotation, then the relay, then the snap, until the chain fires in order without thought. Speed is what you get for free once the timing is grooved.