What good is Taiji quan if it is so slow?

You may have seen mostly elderly people practicing taiji very slowly, and even younger practitioners do it slow. This is the form they are practicing. The form is practiced slowly in most taiji styles, although there are fast explosive forms as well. If one is asking what good is it, I assume you mean in a fight. In actual applications and self defense situations the moves are sped up to actual fighting speed. Of course you can't move so slowly and expect to overtake an opponent with fast reflexes. But the inner peace and quietness gained by training the form slowly, can help one to slow down a situation in one's mind, giving you an opportunity to analyze physical data more clearly. There are many benefits of training slowly, but if you only train the form and never think about fighting applications don't fool yourself into thinking this makes you a better fighter. One has to keep clear that forms are forms and fighting is fighting, forms give you the basics and a chance to analyze your own positions. If you want to fight you must fight and think about actually applying techniques.