Switching speed is dominated by electron mobility (drift velocity) in transistor gates.
We can improve by shifting to faster materials, such as GaAs, or just by making the gates smaller.
How small can we go: what is the silicon end point ?
Rule of thumb: the product of delay and power consumption of a gate is largely constant, leading to a design trade off. (Also called the speed-power product).
Units are the Joule: the energy for a logic transition in the gate.
Total consumption = Gate Power + Wiring Power.
Electric charge in the wiring nets is proportional to their capacitance and hence their length and width.