Zoom Blur rushes your word at the camera out of a smear of motion, decelerating hard so it snaps to a stop rather than easing gently into place.
zb-zoom scales the word down from a large starting size with a directional blur that shrinks as the scale settles — the blur is tied to the rate of change, so it disappears exactly when the motion stops rather than fading on its own schedule.
The speed control stretches it, but the effect depends on rapid deceleration — slowed too far it stops reading as speed and becomes a plain scale.