> If we could run a CPU with no heat generated Nitpick: not much heat generated, but not no heat. There are fundamental lower bounds on the entropy increase ca

> If we could run a CPU with no heat generated Nitpick: not much heat generated, but not no heat. There are fundamental lower bounds on the entropy increase caused by doing irreversible computations (Landauer's principle), superconductors or no superconductors. TAANSTAAFL: the universe won't let you compute for free. As for what limits the clock speed: for one thing the speed of light - information still has to get from one part of a CPU to another.