The slow movement found in taijiquan can be hard to understand from an outsider point of view. It is natural to want to speed the motion up, and it is true that this is done in application, however to speed up the motions can also create terrible misunderstanding about the nature of the use of force and energy in taijiquan.
Taiji is all about the conservation and utilization of momentum, to be able to do this a comprehensive understanding of the nature of force must be had. For example expansion has retraction on the other side of it, this is because there are complimentary pairs which are thought of in terms of Yin and Yang. However do not let this mislead you, there are no things that are yin, or yang, because these terms illustrate a relationship between these two qualities, thus they do not exist independently of each other. Some people may tell you that slow movement is yin and fast is yang, however this not the case because yin and yang are not properties, they are a relationship.
In the use of energy and force there is essentially a kinetic wave involved. In taijiquan this kinetic wave is generated, manipulated and transmitted in a way unique to the art. The methods involved are not such as can be conveyed by any text or image or information. While they involve a profound understanding of force and superb use of body mechanics, this understanding and use cannot be seen or explained. No eye can pick up this comprehension of force.
Involved in this methodology is a form of conservation of momentum that plays a pivotal role in the movements found in the various taijiforms. The aspect of flowing requires this conservation of the kinetic wave and this skill can be quite hard to see. When the movement is slow and continuous the momentum of the techniques flows from one to the next, however when we speed them up improperly something happens that changes this effect and can lead to trouble. When we move quickly our ability to conserve the momentum becomes challenged and we must use extra force and tension to hold back our movements. You can feel this if you punch the air as loose as you can in front of you, but with speed. When your strike reaches the end of the travel the force of it becomes absorbed by the body, it simply cannot push enough air to be able to issue or transmit the force of the wave fully into the air, nor can it simply be controlled and the momentum conserved the way a slow movement easily allows.
The problem with this fast motion in the air is that it trains the body to do the wrong thing. This does not mean there is not a proper way to do the taiji moves fast and conserve the momentum, however the vast majority of fast movements seen in modern schools do not display proper technique in this regard. It may never even become apparent that the movements being practiced do not transmit force, because they still impact with force, however there is a very large difference between kinetic impact and the transmission of a kinetic wave. Kinetic impact for example can be trained for with a heavy bag, this is not the case with the kinetic wave. When you transmit force properly none of it is retained, this requires a type of relaxation that cannot be had with uncontrolled sneeze like movements whose kinetic wave is absorbed by the body. Performing the moves this way looks very impressive to others, however it trains the body to reflexively absorb a great deal of the force of strikes this is both inefficient and contrary to the principals of taijiquan.
By practicing slowly the foundational skills are learned properly, the body learns what to do. By speeding up the movement, particularly for beginners, we can create more problems than we can solve. It can be very hard to beginners to be able to conserve the momentum of their motion in fast forms and this can lead to problems understanding the techniques. This is because faster motion requires more effort and tension.
One common taiji myth is that you can get better at something without practicing it. This myth knows no limit but a common version is that by practicing one so called harder motion, then a simpler motion is learned, even though the simple version is not being practiced. However the body shall always defy such logic and it will simply learn what it does. You cannot learn to transmit a wave of kinetic force by any method other than practicing to do it. This does mean that one will learn to speed things up, and that one must learn real speed timing for the moves, however this should not be done until the slow movements can be done properly, and it cannot be done by practicing the wrong moves fast.
There is the idea that if you practice a form incorrectly that this is ok and then you can get corrections later. This is not true, you will only build bad habits and correcting a habit is much, much harder than learning a technique. Likewise there is the idea that practicing a form will allow you to learn taijiquan, this is also not true. By practicing a form we learn to do a form, doing a form is not practicing taijiquan, it is only practicing a taijiquan form. The form is not enough, and worse the image of the form is far, far too little. The real forces and skills of taijiquan are unseen forces, they cannot be learned through imitating the motion of others. They cannot be learned from a book or video, they require transmission and practice of many aspects of their refinement.