Note that I've never played tennis.
I'm not sure I'd agree with this information. Serves can occur at 100 mph as a conservative number for a maximum speed, correct? So a serve is hit at a theoretical 100 mph and then returned at 30 (?) mph. At the instant the serve is returned, the impulse is quite large, meaning the force on the ball is pretty big. Would you want to get hit with a 100 mph serve? I wouldn't. Why? Because that serve gave the ball quite a bit of force, almost all of which would be absorbed by my body upon impact. This force would be even greater at a time when returning the serve, when the racket would place an even larger force on the ball than the force initally placed on the racket by the ball, causing it to change directions very quickly. That takes a pretty large force.