Slip Control vs Constant V/F Control for 3-Phase ACIM
Slip speed measured in RPM or radians per second does not change with rotational speed. Slip speed is proportional to torque. Obviously slip speed as a percent of operating speed changes with operating speed, but slip speed in radians per second or RPM is what is being controlled.

Controlling slip provides better performance at low speed because the V/F can be dynamically optimized for any given speed and torque. There are methods of adjusting V/F based on operating frequency and current that are much better than just a fixed V/F profile, but controlling slip is better than that.

Slip control would be based on the actual rated slip for a specific motor. That would be determined by motor's synchronous speed at rated frequency as calculated from RPM = 120f/P minus the rated speed at full load as marked on the motor's rating plate. The torque in Newton-meters at rated load is [rated power (kW) X 9549] / [rated torque(N-m) X RPM].