<p>Exciton–polaritons (EPs) are hybrid light–matter quasi-particles that hold great potential for quantum optics and optoelectronics. Bulk EPs, or elementary dressed states, are limited by the small exciton binding energies and oscillator strengths of existing semiconductors, thus restricting their formation to cryogenic temperatures and hindering practical applications. Here we overcome this fundamental constraint and achieve room-temperature control over bulk EPs in a bare halide perovskite crystal featuring an unusual exciton–photon–phonon coupling. Time-of-flight measurements and first-principles calculations reveal an exceptionally large exciton longitudinal–transverse splitting up to ~50 meV that substantially modifies the EP dispersion. From the coherent EP dynamics, we establish that the long-range bulk EP propagation occurs in the ballistic regime. We further demonstrate tailorable slow light with different halide perovskite crystals, with a broad spectral window extending across the visible range. Importantly, our findings reveal the intrinsic polaritonic properties of layered halide perovskite semiconductors, paving the way for their straightforward application as optical delay generators and ultrafast photonic devices.</p>