Lead has less metallic character, so it is a poor conductor of heat. Lead lies in group of the modern periodic table. metallic characters decrease, as we move left to right in the periodic table. Metals are usually the best conductors of thermal energy. This is due to the way that metals are chemically bonded: metallic bonds (as opposed to covalent or ionic bonds) have free-moving electrons which are able to transfer thermal energy rapidly through the metal. Lead lacks this kind of electrons so it's a poor conductor of heat. But compared to non-metal it's conductivity is quite higher.