Isn't tai chi just slow motion exercise?

No. Some of the training methods are slow, and some are not. As the student gains greater skill, their movements become fluid and dynamic. They move at whatever speed the situation demands.

Slow motion movement, chilled out exercise or dance cannot be considered tai chi. Tai chi is more than this. It adheres to certain parameters, guidelines and rules of practice.

When you watch somebody perform Sifu Waller's taijiquan it looks like nothing special is happening, but the power being produced is incredible. Surprising and very exciting. The biomechanics are remarkable; you learn so much about how to use your body in a healthier, more dynamic way.

(Dr Adriana Chilińska)

Intelligent exercise

In the longevity book The Blue Zones, Dr Robert Kane emphasised the fact that many forms of exercise - especially sport - often have adverse side effects that impede long-term practice. 
By contrast, as a milder form of exercise, tai chi can be practiced throughout your entire lifespan, making it a safer, sustainable choice. Tai chi is a fun, balanced way to exercise. 

What is tai chi?

When most people say 'tai chi' they mean tai chi for health. Tai chi for health is a simplified, movement-based exercise adapted from taijiquan (a Chinese martial art). 

Tai chi is intended to improve health and wellbeing through frequent, regular practice using low effort. 

SILENCE IS NOT to be cultivated, it is not to be deliberately brought about; it is not to be sought out, thought of, or meditated upon. The deliberate cultivation of silence is as the enjoyment of some longed for pleasure; the desire to silence the mind is but the pursuit of sensation. Such silence is only a form of resistance, an isolation which leads to decay. Silence that is bought is a thing of the market in which there is the noise of activity. Silence comes with the absence of desire. Desire is swift, cunning and deep. Remembrance shuts off the sweep of silence, and a mind that is caught in experience cannot be silent. Time, the movement of yesterday flowing into today and tomorrow, is not silence. With the cessation of this movement there is silence, and only then can that which is unnameable come into being.
Opinion is not truth; we must put aside opinions to find truth. There are innumerable  opinions, but truth is not of this or of that group. For the understanding of truth, all ideas, conclusions, opinions, must drop away as the withered leaves fall from a tree. Truth is not to be found in books, in knowledge, in experience. If you are seeking opinions, you will find none here.

(Krishnamurti)