One of the fundamentals of singing concerns Slow movement.
There are two main purposes for doing a vocal exercise slowly: 1. so that we can pay close attention to what we are doing, and 2. so that we build up our muscular stamina and endurance.
When we do an exercise slowly, our attention is drawn to each and every movement. We focus on the feel of healthier action, and make discoveries as we carefully explore the correlations between cause and effect, between what we are doing and what happens as a result.
When we do an exercise slowly, it is like going to gym and lifting a weight slowly. It is quite difficult. As we lift the weight slowly once, we might be okay. However, after the fifth and sixth and seventh time, lifting that weight slowly will be very taxing. Our arms might start to shake and feel like we just cannot do it anymore. Contrast that with lifting the same amount of weight, just quicker. To be honest, it feels easier to do it quicker.
Singing through the vocal exercises slowly is important for building muscle which will increase stamina and endurance; allowing us to sing longer and with ease.