Move slow to allow the body to harmonize and release stagnation.
Why is Taiji done slowly? Moving slow allows your body and mind to connect. Its that simple! Slow movements allow you the time to feel what the hell you are doing. You get the time to study minute changes in muscles, body-weight distribution and more. To learn to calm your breathing allongside the changes while moving is also important.
Another part is that moving slowly helps you to root the body as many people float. The stress of daily life makes the Qi of the body rise up. This uprising raises and tenses the musculature.