When training with a punching bag, it is very common to overemphasize power and forget about speed. This is probably because most who buy a punching bag do so for the power; since it is a common misconception that bigger and stronger is better. If a 300lb guy can only throw one punch every two seconds while a 200 lb guy can throw four punches with half the intensity in the same timeframe, I'd put my money on the 200 lb guy. Punching speed is not something that should be second to power, but regarded as equally important.

Pick a spot on the bag and slowly throw punches. Gradually increase the frequency of punches until you are maxed out and stay punching at this maximum speed for about a minute. Rest and repeat. Yes, this is ridiculously simple objective, but very effective. One can also attempt this speed exercise using the weighted gloves and punching the air. It is a common mistake to use weighted gloves on a punching bag; this can cause a lot of damage.

Try throwing punches at a speed that feels comfortable for a couple minutes, and then repeat the above exercise increasing punching speed without stopping. After another minute, go back to punching slower again and repeat. The idea here is simulating a real fight. In reality, you're going to get tired and your punch speed will diminish for a little while until you catch your breath. Doing this will give you some insight into how long you can last before you crash and the hope is that there will be no need for a rest period over some extended period of time.

Many people give up too easily when it comes to building punching speed. It's something that will take time to achieve and something which should be looked at as goal oriented. Instead of just showing up at the gym and telling yourself that you'll do as much as you can do, try setting a goal of 10-15 minutes a day where you gradually increase punching speed. Anything in life that's worth anything requires work and if you have the will and concentration, you'll find a way.