https://images.anandtech.com/doci/13401/2018-10-08%2010.38.03.jpg https://images.anandtech.com/doci/13401/2018-10-08%2010.38.0... The lid is made of copper and

https://images.anandtech.com/doci/13401/2018-10-08%2010.38.03.jpg https://images.anandtech.com/doci/13401/2018-10-08%2010.38.0... The lid is made of copper and as you see there are additional layers. Plus the interfaces between the materials are also limiting thermal conductance. As far as material science goes there's a lot of room, it's just not practical to throw liquid nitrogen cooling or diamond substrates at the problem when you're making a mass market product.