[The fastest person in the room, sure, but nowhere near as fast he is normally: not fast enough to pick Ciel up off the ground and put him down out of harm's way before the chandelier has even hit the ground.
Not sturdy enough to have the chandelier chatter on impact with his body as he shields Ciel with it, either.
Sebastian throws himself between the chandelier and Ciel-- landing hard on the ground, it scoots them across the floor enough not to be immediately under the most crushing weight, but even so Sebastian takes most of the impact as the entire metal construction comes crashing down over them, crystals scattering everywhere.]