I don't think his particulary slower than Shogun and where did you get that toughness comparison? Mousasi hasn't been rocked before. Never been finished due to strikes before and is no slouche on his back. The problem with him is he goes to fight, not to win a contest. Hell, he did more damage on his back to King Mo than he did on top.
He's slower mentally, Mousasi's problem is he has technical ability but he does not have the right mind to be a champ. He plods and moves his feet slowly, Shogun throws faster combos and he has the ability to pounce when he smells weakness. Gegard has all the tools but he doesn't put them together very well, it's like he'd be a good kickboxer and a good grappler but he's not an elite mma fighter. Don't get me wrong, he's a great fighter but we're talking about matching him up against JBJ who would tool him worse than he did to Bonnar.