It is one of the fundamental dichotomies of martial arts - why do we have to practice Tai Chi slowly? When I first started training, a number of my friends were curious. Some were downright derogatory.
They could not rationalise the slow, contradictory elements of Tai Chi with the realities of fighting and self defence. To the layman, it must seem unusual to move so slowly. So why are the movements slow?
The answers are more complex than those implied from first impressions. Slow practice from a beginner's perspective allows them to feel their balance, posture and the transfer of weight between the feet. As you get further on, the slow movements allow you to condition the strength of the legs and to relax the muscles that you are tensing unnecessarily, lowering your centre of gravity and creating a stable frame.
Further on, the slow movements allow you to feel the force as it is transferred into, through and outward in a spiralling manner through the body. Slow movements with partners also reinforce this spiralling, and teach you to become sensitive to your opponent's strength, balance and intention.
Contrarily, you can also perform Tai Chi quickly. There are special sets in Chen style Tai Chi that are intended to be done quickly, known as Cannon Fist. They are for advanced students who have learned the correct movement and know how to relax properly. The required quality of strength, posture, movement and relaxation for Cannon Fist can take much time to perfect. But once acquired, it's never forgotten, and can be done well into old age.