Racket Head Speed is definitely the number one factor in an effective kick serve. The pros swing just as fast or even faster for a kicker than they do on flatter serves.
Racket Head Speed = Energy
That energy can be translated into either spin or pace, one is going to cost you the other.
Granted this relationship is not linear and there is a point where your racket is merely glancing at the ball, which creates decent spin but wastes most of your energy. You need to find the right balance, otherwise you'll have to build up your swing speed.