Yes, it takes a little bit more energy to heat up the mass of the cast iron than a thinner pan. Maybe you should have everyone cook by bringing things to temperature, then tossing into an insulated container. that's even less energy spent!
Or better yet, an insulated slow cooker, so there's super low levels of energy loss to convection and radiation!
Yeah, that's the ticket! The environment will thank you if you cook on your car's engine block right after you get home from work, and recycle all your light bulbs.
I guess I'm joking. Cast iron's great for even heat. Though, some day I'm buying an aluminum block.