RC delay with varying voltage explanation requested

[klwdesigns]

Connect a series of gates in a loop to form a ring oscillator.
Odd number of inverters, say 21, 31, 61,,, etc.
supply voltage 3VDC
use o-scope to measure period.
calculate frequency
determine delay per gate.

increase voltage by 1V
repeat analysis

continue until supply voltage is 12V.

So the question is,, why does frequency increase as voltage increases?
I searched this forum and wikipedia but did not find this answer.
I'm sure the answer is staring me in the face so please explain in layman's terms along with scientific notation if you will.

 

[martine1212]

Some Transistor parameters are voltage dependant.
So the speed of the circuit is affected by the change in supply voltage.