There is also another fundamental limit, the speed of light.
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.
It's very easy to design a CPU such that signals do not have to cross it in one cycle. The speed of light is unlikely to ever cap the speed of a CMOS processor.
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.