He's actually a good one on one defender, like I said the main issue is him being late on rotations where u don't have to be quick on. Hell al jefferson and zbo aren't good defenders either.
He's late on rotations cause he's not very quick on his feet, it's also why he gets his shots blocked so much.
Jefferson has been on only 1 good defensive team in his career, and Zbo has 3 plus defenders on the floor with him. Just accept that there's always going to be some defensive spillage with Kanter, you should have more than enough firepower next year to be right there with the Warriors/Cavs.