The input frequency of a servo
Typical RC servo frequency is 50 Hz, as you can see from the SG90 datasheet. This question states that servos are pretty forgiving about the frequency, but why not use the right frequency? Note that the servo input controls the position (angle) not the speed. Also, the pulse width (amount of time on) matters, not the duty cycle (% of time on).

(Edited to incorporate comment feedback.)