Summary: Use Frozen Rotor for solving rotating analyses.
Frozen Rotor reduces simulation time and produces an approximate solution. When enabled, the rotating region does not rotate during the simulation. Rotation and associated momentum terms are imparted to the flow.
Frozen Rotor is useful for quickly simulating rotating devices without the complexity of numerically rotating the physical rotor. Note that the interaction between rotating and non-rotating objects (such as rotor-stator or impeller-volute) is not predicted.
Frozen Rotor analyses are generally faster. Because the rotor is static, you can use larger time steps which lead to a steady-state solution in less time.