An InP L3 photonic crystal cavity on a suspended membrane has a characteristic time of similar to 1 mu s, which is 5.4x slower than the thermal cavity lifetime. To explain this mismatch, we use a lumped circuit model containing not only the cavity temperature, but also an average temperature of the membrane. Comparison of this model to a 1D COMSOL simulation, results in a thermal time constant of the membrane that is 7.9x slower than the thermal cavity lifetime, and slows down the transient response. The membrane behaves similar to other membrane cavity types.