If, by "asynchronous motor" you mean an AC induction motor, then the important detail is that the rotor lags the AC frequency (e.g., 1750 rpm for 60 Hz drive, instead of 1800 rpm).

The amount of "slip" depends on the torque: as the load torque increases, the rotation speed falls.

Therefore, you can change the speed by changing the AC frequency, as in a variable-frequency drive, but the actual speed is not directly determined by the frequency only.

See the basic description:

https://en.wikipedia.org/wiki/Induction_motorOn the right side of this article, there are several curves showing speed (horizontal axis) vs. load torque (vertical axis). At zero load torque, the speed is "synchronous" (i.e., 1800 rpm for the example above), and the speed falls off as the torque increases (right side of the graph). Ordinarily, motor operation is on that part of the curve on the right where the speed is a monotonic function of torque.