Why is propagation delay a function of supply voltage?
I didn't really buy this argument. All else being equal, simply increasing the supply voltage should not increase the small-signal gain, which is out of scope anyways since the 'amplifier' is being over-driven with digital signals.


It really does. Don't think in terms of voltage gain, think in terms of output current:

When you increase the supply voltage, the gate capacitance of a FET is charged quicker, because exponentially more current flows through the involved semiconductor junctions.

That leads to a quasi linear increase of maximum switching speed in modern CMOS ICs. Sadly, you buy that with a raising energy dissipation per switching (=heat!), and thus typically in a superquadratically effort needed to keep the IC at acceptable temperatures. That's one of the main reasons (if not the main reason) there's little complex logic clocked at 10 GHz.