“In the form it is slow but in usage it is much faster.”
This is totally erroneous! One of the most important aspect of taiji is that it is used just the way it is learned and practiced. If you train slowly in the form, you must use it slowly in application. Otherwise, you are making it up along the way. The purpose of any training is to get yourself ready for similar combat or self-defense situations. You never train “this” but do “that”.
But then why are some taiji moves slow? It is complicated to explain in words here but in a nutshell, the slowness is only an illusion that the average viewer gets. In reality the seemingly slow movements are not slow at all. This is the secret of taiji as martial art. Taiji used many skills, techniques, qualities, structures, dimensions, etc. to accomplish a job. For those who live in a single dimension (only this three-dimension) world, these movements will only appear to be slow.