Experiments reveal that the measured group velocities of molecular polaritons are lower than those predicted by their dispersion relations, which is referred to as the renormalization of group velocity. Ultrafast measurements reported in Refs. 1 and 3 indicate more deviation from the predicted velocities with an increase in the wavepacket's excitonic character. These findings underscore the importance of phonon-mediated scattering in influencing polariton transport via its excitonic component. Additionally, Ref. 2 reports that the polariton wavepacket's velocities are strongly dependent on the cavity quality factor Q, although Q is not related to the polariton's dispersion. The Q-factor, which affects the polariton's lifetime, also correlates with the polariton's coherence lifetimes and hence, its transport properties, including both transient mean-square displacement (MSD) and group velocities.