Well, very few people ask this question. It's just what they expect, that tai chi should be slow, but the why is important. Doing it slowly allows you to feel where the weight is with each movement, which helps with your balance. More than that, it also strengthens your legs over time, just as much as hard exercise, but in a low-impact way. Then again, over time you have the opportunity to develop mindfulness:

Think of riding a bike - when you first start, everything not only feels strange, but you are very aware of what is happening with your balance and that's your mindfulness in action. As you get more confident with cycling, you can find yourself going into automatic pilot mode as you weave your way through traffic at a faster speed than when you started.

By doing tai chi slowly, you retain the opportunity for mindful noticing what is happening in every movement, which is why tai chi is referred to as "meditation in motion". This allows you not only to get the health benefits of tai chi, but also to de-stress and learn real relaxation. So keep practising slowly and enjoy your tai chi!