It had been proposed that the microcavity effect could enhance the spontaneous emission rate and hence dramatically increase the modulation speed. However, the work in this paper reveals that the situation might not be completely correct for the 1.3 μm GaAs based quantum dot (QD) photonics crystal (PhC) nanolasers due to the complex carrier dynamics and close hole levels. Based on the all-pathway rate equation model considering the carrier relaxation dynamics, the influences of quality (Q) factor of cavity on the threshold and modulation responses of 1.3 μm QD PhC nanolasers were studied. It is found that the high Q factor can improve significantly the threshold of QD PhC nanolasers, but it also increases the photon lifetime and deteriorates the modulation bandwidth. Hence there exists an optimized Q factor (~2500) for the nanolaser with a modulation bandwidth exceeding 100 GHz. For the energy consumption, the best value corresponds to a Q factor of ~7 000. So an overall consideration is preferable in designing PhC nanocavity for both high speed and low energy consumption operation of QD lasers.