Fig. presents the energy diagrams for the UP and LP bands formed from hybridizing the photonic band and the excitonic band. The LP and UP bands are color-coded based on their photonic character. The collective light-matter coupling strength is √ N g c = 120 meV. These polariton states are analytically expressed in Eq. S13 in the Supporting Information. The initial excitation Fig. shows v g with different initial energies (corresponding to different k ∥ in Fig. ). Here, the cavity is lossless, with Γ c = 0. The solid black line indicates the group velocity obtained as v g = ∂ω -/∂k ∥ . The open circles with different colors are v g for the excitonic system with different reorganization energy λ. As λ increases, v g decreases, indicating that the polaritons propagate at a reduced v g due to increased exciton-phonon coupling, which was referred to as the group velocity renormalization. Increasing λ or increasing the excitonic character (increasing E) causes more renormalization of v g , in agreement with the results in Ref. 3. Fig. illustrates the effects of cavity loss on v g by changing the Q factor (loss rate Γ c ), with fixed λ = 36 meV. With a decreasing Q (increasing Γ c ), v g further decreases due to rapid attenuation of the photonic contribution to the polariton wavepacket. Fig. presents the impact of cavity quality factor Q on v g with a broadband excitation on the UP band (indicated with the gray Gaussian wavepacket in the inset of Fig. ), to model similar experimental conditions in Ref.