Slow is smooth, smooth is fast is the mantra at my current ski school. What we mean there is not to rush beginners through the progression. If you move ahead quickly, students don't master each drill, and then struggle on later tasks. If you spend more time on the early tasks, and really master the basic movements, then they'll progress quicker overall as they struggle less later in the lesson. But can that same mantra be applied at all levels of skiing?
Yes, of course. We can all use more time practicing the fundamental movements of skiing. To become really good at skiing, you need to balance skiing near the upper limit of your ability with skiing easy terrain where you can concentrate on making movements properly every time. A perfectly executed run down an easy slope will train you better than a gnarly run where you just barely hold on.
In training, speed is not your friend. Imagine riding a bicycle - the slower you go, the harder it is to maintain balance. Furthermore, at high speeds its more difficult to sense what exactly you're doing. Many skiers will move their legs sequentially, rather than simultaneously, with the outside leg releasing the edge before the inside leg, and thus beginning rotation before the inside leg. At high speeds, there is almost no time between the two movements, and it will feel correct - but at high speeds, even a little flaw that you can't feel will still make a big impact on your skiing. Slow the speed down, and you can sense the little flaws in your skiing and work on them.