We developed an efficient approach for investigating polariton transport with quantum dynamics simulations. We achieved quasi-linear scaling for our quantum dynamical method, enabling simulations of N = 10 4 molecules collectively coupled to M = 10 2 cavity modes in a GHTC Hamiltonian. The results from quantum dynamics simulations confirm the v g renormalization effects and demonstrate the v g reduction due to excitonphonon coupling and cavity loss. Furthermore, the transient MSD of polariton wavepackets with broadband UP excitation demonstrates transient growth and then contraction, agreeing with the experimental observations in Ref. 2. This is due to fast expansion of the UP polariton wavepacket in space and the relatively slower rate of transitions to the dark exciton wavepacket, as demonstrated by our quantum dynamics analysis (Fig. ). Finally, from the transient MSD, we were able to analyze the transport characteristics of the wavepacket that illustrates a ballistic-to-diffusive turnover, which has been experimentally observed in Ref. 3 and Ref. 1. Overall, the results from our quantum dynamics simulations successfully capture all the trends observed in recent polariton transport experiments. The current theory does not consider static disorders, inter-molecular interactions, or Peierls phonon (that fluctuates the intersite couplings). Their influence will be explored in future work.