The speed at which boiling water kills an animal is directly related to the ratio between the contact surface and the volume. For usual crabs and lobsters the

The speed at which boiling water kills an animal is directly related to the ratio between the contact surface and the volume. For usual crabs and lobsters the volume is low enough and the contact surface quite large, their bodies are not sealed, boiling water enter directly inside. I think it can take a few seconds longer for huge crabs, especially if the water volume is not so large. If you cut the head of a lobster, I think it can suffer for an extended time before dying, but not if you cut it in half longitudinally, as this is usually done when cooking them grilled. I've carefully observed the process, and there is no question about it.