1. A method for optimizing power usage by individual subsystems of a portable computing device, the method comprising: determining a plurality of voted voltage values for a plurality of subsystems within the portable computing device, wherein each of the plurality of subsystems is associated with a shared power domain such that each of the plurality of subsystems is supplied a same voltage level dictated by the shared power domain and at least one of the plurality of subsystems is a multiplexed subsystem comprising a plurality of processing engines; determining a reduced set of voltage values based on the plurality of voted voltage values, the reduced set of voltage values comprising three voltage levels; calculating an energy efficient voltage level from the reduced set of voltage values comprising the three voltage levels; determining that the present voltage level exceeds the energy efficient voltage level; then performing the following in sequence: determining an energy efficient operating frequency and power collapse duration for each subsystem based on the energy efficient voltage level while each subsystem is operating at the present voltage level which exceeds the energy efficient voltage level; setting each subsystem to its energy efficient operating frequency while each subsystem is operating at the present voltage level which exceeds the energy efficient voltage level; identifying a subset of active processing engines from the plurality of processing engines; setting the shared power domain to the energy efficient voltage level; and processing a workload allocated to the multiplexed subsystem with the subset of active processing engines at the energy efficient voltage level, wherein processing the workload with the subset of active processing engines increases power usage efficiency of the multiplexed subsystem.