Speed and accuracy are inversely related. In other words, the faster you try to go, the less accurate you will be in your movements. This relationship always exists, no matter how well trained. However, the slope the line can and does change as someone becomes more well trained. So, for example, a really skilled martial artist will be able to place a strike at high velocity far more accurately than an untrained or moderately trained person will be able to do. However, even for that well trained person, if they were to examine their own accuracy at their top speed versus lower speeds, they would find that they are more accurate when they move slower.
Long story short, one needs to do both. Slow work to develop precision and accuracy is a must. Plus, movement patterns can be learned correctly in a way that is harder to do if you start off doing everything rapidly. At the same time in one's development, it is important to work on those same movements at speed (safely by the way), so that you develop the ability to perform with greater precision at higher velocities.
Then, one needs to consider what motor learning people refer to as Environmental Regulatory Conditions. Specifically, when someone is trying to prevent you from performing the movement. In martial arts we typically refer to that as sparring or training against a resisting opponent. If one ever hopes to be able to execute their skills reliably against another person, this step is crucial.
This is a motor learning/control issue, and it is going to take a ton of reps to get there.