Many fighters find that, as they progress in training, they need to work on their punching speed. It is critical to keep punching speed in mind while training, since it doesn't matter how strong you are if your opponent is faster than your punches. Here I will go over some simple techniques to improve your punching speed.
First you will have to practice for speed. You will have to train while keeping in mind your focus is to train for punching speed. Weight training is fine, but in general you will want to stick to training your core muscle group (the muscles that connect through your abdominal, chest, and back). Upper body strength is important for a strong punch (just as leg strength is) but it is the core muscles that help you to speed your punches. A lot of people try and use weights or weighted gloves to build punching speed, but they hit a wall where they can't go any faster. That's when it is time to drop the weight and focus more on repetitive punch training.
Another trick that helps to break through a possible plateau in punch speed training is to learn to relax. When they study the brain patterns of different athletes while they are performing they find that as the moment of impact the brain actually appears similar to when an individual is meditating or near sleep (this is known as delta brain waves). When your mind is tensed (alpha waves), your body is too and it will react more slowly than if it, and you, were relaxed. Breathing exercises, meditation, and even yoga can help improve your punching speed when you have tried everything else.
When training for punching speed, try and start slow and build the speed gradually. This will help to ensure that you do not lose your technique as you increase your speed. This was somewhat difficult for me at first as I was so concerned with doing it faster, strong, and harder that I was losing sight of the real focus of any training, to perform better. Use a speed bag, of course, to train punching speed but also to train the necessary endurance to keep that speed up throughout the fight. When working a heavy bag, move your feet. You’re not trying to power into and through the bag, as you would normally, but to land several solid and quick punches while adjusting for the swing of the bag by moving yourself into striking range.
If you strengthen your core muscle groups, remember to stay relaxed and move your feet, and of course practice consistently you will find that you can continue to build your punching speed.