Because of the special relativity theory nothing including electrical current can transmit information faster than the speed of light (c = 3*10^10 cm/s). So if maximum size dimension of your CPU is say 2cm (I'm ignoring that they actually can have multiple smaller cores), then the maximum physically achievable frequency is upper bounded by size/c = 15 GHz.

Although it has not been reached yet, it has the same order of magnitude as the record frequencies achieved by overclockers using liquid nitrogen cooling (something a little below 9 GHz). Also it shows that reducing transistor size and as a consequence overall CPU size increases maximum physically allowed frequency.