The time it takes "to complete one cycle" is a perfect definition for <em>period</em>.
So the <em>2 sec</em> that you measured is the swing's period.
Frequency is just 1/period (the 'reciprocal' of the period).
For this swing, the frequency is 1/(2sec) = 0.5 per second = <em>0.5 Hz</em>.
( "Hertz" means "per second")
OA the speed will increase four times
