Can I make a distinction between "friction" and "effort"? If you're riding a bike up a hill, you can't go up without effort. But not all of your effort is actually moving you up the hill -- some of it is being lost in friction: inefficiencies in your muscles, friction in your gears and wheel and chain, wind resistance. Similarly, you can't learn anything without effort; but it's often the case that effort you put in…
But I think at a more personal level truly learning and having that learning last happened because friction not because it was effortless. I honestly don't remember much about things that were effortless. Things that don't take effort can't show you what your doing wrong because it was effortless, there was no resistence to feel where you might be going wrong. I agree there is a optimal point of friction but minimization might not be that optimal point.