If you are building muscle, like a body builder does, emphasizing Muscle Size/Hypertrophy and Muscle Definition, then, yes, that will get in the way of your punching.
However, if you train specially for Strength Gains, this will Add to your Punching Power and will NOT diminish Punching Speed. Training for Strength will also lead to some Muscle Hypertrophy but will look very different from a Bodybuilder’s Muscles.
Also, your muscles are predominantly Fast Twitch, Slow Twitch etc, due to Genetics and will remain the same, before and after Strength Training. Muscles will not change their type due to Gaining Strength.
If anything, your punching speed will increase due to significant increases in Muscle Strength.
As long as you do not Increase your Food Consumption by an Inordinate Amount, leading to a LOT of unnecessary Body Fat Gain, then, Strength Training would NOT be detrimental to your Punching Speed.