What factors determine the maximum CPU clock rate? A 6502, for instance, clocks in the megahertz range, while an Intel x64 chip typically clocks in the gigahertz range. 

All things being equal, if the 6502 is manufactured using the same materials, nanometer rescaling, and the same photolithography process as the latest x64 CPU, it's conceivable it should also be capable of running at gigahertz speeds, especially considering the 6502's simpler design and lower gate count. 

Am I missing other factors, such as the CPU design, which could limit its maximum clock rate?