The energy efficiency can mainly be increased by improving the heat transfer from the fire to the pot. Increasing the heat of the flame or gas contacting the pot can increase the heat transfer. (Design priciples for wood burning cookstoves)
When the cookstove is made of clay, the large mass of clay will absorb a lot of heat, which makes the cookstove less energy efficient. To make it more efficient the combustion chamber should have a layer of insulating materials on the inside, preferably with a small layer of still air in between the clay and the insulating layer.