The Poster (muy thai fighter) is correct in a sense , but only correct in that you dont see much fighting applications taugt with tai chi. It is mainly used for relaxation/stress reduction, and for internal work. You just dont see many modern tai chi fighters.

but i also think that fighting is not all that Tai chi is. The internal road is a longer road to walk and at the beginning you will lose fight confrontations. Once you get past all that fighting nonsense anyway you will find a new level of your training and then you will discover that fighting will come easy to you.
But Taichi is a fighting art and its done slowly to convey the techniques and when performed in sparring it is very fast.
but i have also seen Muy thai Boxing and it is a brutal fighting system. if i were to see a tai chi practitoner and a muy thai practitioner each with say 3 years training i would put my money on the muy thai fighter(hands down).
advance the training to over ten years and i would put my money on the muy thai fighter hands down.
advance the training to over 20 years and my money goes for the tai chi practitioner hands down.

Peace,,,,WindandMountain