The more densely packed computing circuitry becomes, the fast it can run, in part because the connection paths are so short. Until room-temperature superconductors are invented, passing an electrical current through a physical connection, no matter how short, will prevent the current from running at the speed of light due to the phenomenon of electrical resistence. Another problem arises as a result of densely packing circuitry: Heat. Lots of heat.

This is a problem that can't really be eliminated, because heat is a natural manifestation of entropy. When you lose decoherence in anything for any reason waste heat is generated …