The reason for limited thermal performance is that electrons scatter more when wavelengths associated with the electron wave function are on the same scale as the gates. Also, tunneling becomes more of a problem at these length scales. Electron scattering (bouncing off of) an impurity causes heating because electrons are massive particles with a significant momentum. The electron imparts its energy to the impurity in the form of heat.

When CPUs use graphene, we should see a 10-fold jump in CPU clock frequencies, because this material is a better conductor that scatters electrons less. Further down the road, classical optical computation may provide a more energy-efficient way compute since photons are massless particles with relatively low momentum. But there are many challenges associated with using light for computation; one is that you can't hold light on a capacitor like you can with electrons.