Why Taiji had to be practice so slow

Ever since I took up Taiji, this question had always come across my mind.
Why Taiji had to be practice so slow?

I had gathered many answer, no right or wrong, but it does make sense.

1) Walking/Moving mediation. Practice Taiji as a form of moving mediation.
No other thoughts and no distraction, just you and your body.

2) Slowness and repetitions let your body remember the finer points sub-consciously.
Many Taiji passages talk about the body like a threaded pearl, each section connecting
to the next. It also mentioned about the mind (yi) leading the energy (chi). 
Hence, though slowness and repetitions, the body remembers how it should move. 
When it comes to real application, the movement may be fast, but the body moves correctly
to channel the energy (chi) to the desalinated area.