I think there's a few points where drill makes sense. In each case, after you have a good understanding, and you're seeking to become fluid and avoid dumb mist

I think there's a few points where drill makes sense. In each case, after you have a good understanding, and you're seeking to become fluid and avoid dumb mistakes. (Even if you have a good understanding, if you're consciously thinking about each step you'll error a fair bit, and on a 20 step problem you can't afford this error rate and slowness) Arithmetic facts, basic algebraic manipulation, big polynomials, derivatives and integrals. But we do much more drill than that. And, worse, it's often as homework, where you can be guaranteed a decent slice of the class is practicing it wrong.