Most people equate tai-Chi with slow movements however this is just the inital stages, once a student can move slowly correctly then the speed and power of the movements can be increased without losing the essential structure (this is the internal bit of internal martial arts).

The clip below is my teachers, teacher Nigel Sutton performing the Fast Tai Chi form at the opening of his new training hall in Malaysia. Notice the vibrating or shaking at the end of each move, this is known as Fa-Jing or short force. It's a speciality in Tai-Chi, whole body strength concentrated within a very small movement.