Generally speaking, the power of the microwave oven is measured in watts. The microwave ovens with higher wattage can cook food faster than the ones with lower wattage. But wattage is not the only factor for cooking speed.
The design of the magnetron, the material used for the cavity wall, the shape of the cavity and the turntable, the number of power levels, the waveguide, etc., are also important factors for cooking speed.