Punching a medicine ball is not really going to help you achieve your stated goals.
The reason people throw a medicine ball is because it requires explosive forward motion from the arms, which can help develop explosive arm strength. (Folks also do "jumping" pushups to get a similar effect). Of course, a punch isn't just the arms, it's also the torso turning, and footwork, etc. However, a medicine ball does allow people to develop some strength and power with the arms in this way.
Punching a medicine ball, however, doesn't necessarily give you strength building, nor speed, anymore than... I guess punching any other random object you can find.
Generally with punching targets you are looking at:
Hard vs. Soft (and safety for your hands in the process)
Weight, if the object can swing or recoil
Shape, and ease of delivering appropriate strikes to it (compare, for example, the classic heavy bag vs. the uppercut bags).
If you're worried about punching speed, find something you can punch safely and put force into it without hurting your knuckles or wrist (even if that turns out to be something like a towel hung on a clothes line) and focus on your form and coordination. Also realize that part of punching speed is your footwork and ability to generate force through your legs and torso, as well as how little you telegraph your attack (which is where you see people who look "slow" if you watch them fight, but they're constantly getting hits in if you're IN the fight... it's hard to read if you're the target.)
If you're worried about power, then look to targets that provide more resistance and fit the factors I mentioned above.