First of all, Tai Chi is executed quickly when applied in combat. It is practised slowly to develop your inner strength. When it is applied in combat, it should be free of any muscular strength at all, utilizing only tendon, ligament, and through opening you qi pathways. Also important to note is that not all Tai Chi systems are combatitive. Chen is the eldest and most difficult system, and the most martial based. The other systems of Tai Chi, such as Yang and Wu, were developed from Chen and focus more on overall health, by maximizing qi flow. So to answer your question, it depends on the attitude of the Tai Chi practitioner. If they focus on the combat direction of tai chi, then this question becomes just another "insert style vs. insert style". And Tai Chi can be an extremely powerfull martial art if that is its aim. I hope this helps.