It's not about speed, it's about technique, toughness and experience. Walker is a soft 1-on-1 defender, and Jokic will just flat out toy with him near the perimeter.
Besides, in his current form Kessler should stay near the rim anyway. He's by far the most useful there. I'm not crazy about the idea of putting Lauri on Jokic because it takes away from his offense, but that might still be the best option. As said above, Jokic will get his against anybody, but Lauri as the 1st line of defense and then Kessler at the rim may be the most effective combination.