We saw in the latest chapter, Law was able to activate Room before a charging Gear 2 Luffy got very far. It's not like you can say he wasn't going at at least a decent speed either, because even though he wasn't intending to hit Law, he was intending to hit Doffy who'd be in that very same spot, meaning he'd have to have been moving at enough speed to build up enough energy and momentum to damage Doflamingo.