If the voltage is reduced, the torque capability will be reduced at any given slip is proportion to the square of the voltage. If the voltage is reduced to half, the torque at a given slip will be 25% of the previous torque. The slip at any given load torque will increase accordingly as shown below.
The motor will operate at the intersection of the torque capability curve (blue) and the torque demand characteristic curve of the load (red). The torque characteristic curve of a fan is shown because this type of speed control is sometimes used for fans. Loads that require the torque to remain constant at reduced speeds or have less reduction in torque requirement are rarely operated this way.
The losses due to slip are proportional to the load torque multiplied by the percent slip.
This can not be done with a single-phase, capacitor-start motor because the motor needs to be operating near full speed for the centrifugal switch to disconnect the capacitor.