I have put on a lot of size in the past 2 years, but unfortunately I have gotten slower. (punch wise)
How can I develop a faster punch?
Your problem is that you are training for strength and size (i'm guessing). You need to train for power(Force x velocity). No matter how much you can lift, if you cant do it fast enough your punches wont be effective.
To train for power do three sets of 15 to 25 high-speed contractions with 30 to 60% of maximal resistance. If you use free weights be very careful and make sure that eventhough you are doing high speed contractions keep good form. The key is light weights and fast contractions.
stretching and becoming really flexible i imagine would help.
speed in a punch is more of fluidity...but, mass is more of an impact.....