Dumbledore wouldn't have used it as an excuse to torture Harry to insanity like Snape did, I think Snape was intentionally doing a bad job of teaching Harry Occlumency because it gave him a personal excuse to torture Harry without using an illegal unforgivable curse that would get him landed in Azkaban.
Snape’s just a bad teacher. He thinks that because he is good at things, his students should be able to pick those things up easily with some basic instructions. It’s the same with potions. He just puts the recipes up on the board, and then gets mad at students who aren’t intuitively good at following those instructions.
^ Hermione aces nearly everything in assignments and gets all OWLs (except for maybe in Divination, but that's it), and Snape treats her practically worse than Harry.
