The effects on torque/slip when changing speed?
Slip is determined mostly by the torque delivered to the load.

For most induction motors, slip at rated full-load speed and load torque is about 2 or 3 percent of synchronous speed. For the various numbers of poles, the 3 to 3 percent holds true, but the slip RPM changes because the synchronous speeds are different. Below rated torque, the slip is lower and fairly linearly proportional to torque.

If the frequency and voltage are changed proportionally with a VFD, the slip RPM for a given torque is the same as for operation at rated frequency. VFDs generally adjust the V/Hz slightly to maintain the same slip RPM/torque.

If the voltage is reduced without reducing frequency, the slip at a given torque increases approximately proportional to the square of the per-unit voltage.

If the frequency is increased without increasing the voltage, the slip increases approximately proportional to the square of the per unit V/Hz.