The heat is all being dumped into the living space either way, so you aren’t losing anything from the metal. It takes away from how much heat will be available in the earthen part after the stove is turned off, but it still warms the room. Plus I imagine it’s nice to have some heat immediately instead of having to start the fire hours before you actually want it.
You could adjust the design if you wanted to change the ratio of fast heat and slow heat, but you would always need to have some part with a high thermal conductivity.
