Speed is something that to a great degree we are born with but it is also something that can be developed and trained for to a certain degree. Body size and build are a large part of determining this. Improving strength through training and light weight lifting can improve speed. One of the most important aspects in developing speed that can be learned is the ability to relax and be loose. If tension is present physically, the body becomes stiff and one is fighting one's own muscle tension to move. This greatly slows the speed of the fighter down. By learning to relax and only focus muscle energy when necessary for the execution of the attack or defense, the body is able to move much faster. Tension is then called on at the point of impact or on the muscles specifically used to propel the limbs forward in an attack or used in a block for a defense. Muscles not necessary to execute the technique are relaxed and loose. This also has a great advantage of conserving energy. Body tension consumes energy from the body even if the body is stationary. This is why so many karate novices are exhausted when training whereas the black belts are hardly breathing hard given the same technique to perform. This is even the case if aerobically they have the same level of physical conditioning. Training and experience teaches the karateka to relax when fighting and to exert energy only when necessary.
Speed can be improved by making a conscious effort to relax the body when sparring or practicing technique. At first this may seem almost impossible, but over time the body will learn to be more and more relaxed thus increasing speed.
What is often times referred to as speed has nothing to do with how fast a person can move but more importantly on how fast a person is able to react. To the opponent you may seem like you are moving fast, the truth of the matter may be that you may not be any faster than your opponent, but rather you are able to react so fast to the opponent's movement that your block and counter attack may seem faster. This has to do with reflexive development and again can be obtained through repetition with a partner to the point where a reflexive reaction is developed to any given attack that may be delivered.