Have you ever noticed that when you shorten your punches, your combinations seem much quicker? It's not uncommon to see some guys using only 2/3's or half of their reach while shadowboxing, perhaps they do it for the same reason that it makes their movements feel faster.

Shadowboxing is typically reserved for techniques and tactics work, but for focusing on speed would it be more beneficial to work within a shorter range of motion? This is taking from the concept of overspeed training. The theory is that with repetition, the neuromuscular system will adapt to these quicker movements and your punches in general will become faster. Note, the technique of the punches remains the same with the only difference being the distance in which your arms are extended. This mode of training can be used in conjunction with elastic bands, and can be applied to shadowboxing as well as bag-work.