Why we practise slowly
Speed hides things. A slow class shows you exactly where you are holding on.
A fast class feels like exercise, and there is nothing wrong with exercise. But speed hides things. Move slowly into a pose and you find out, quite precisely, where you are holding on.
The slow classes here are not easier. Holding a lunge for two minutes is harder than holding it for ten seconds. It is a different kind of hard, one that teaches you something about how you stand.