Compared to scintillator based PET detectors, semiconductor detectors tend to provide excellent energy and spatial resolution. However, due to the relatively slow charge carrier collection time limited by the carrier drift velocity, semiconductor based detectors have poor time resolution in the range of several nanoseconds or greater. Possibilities exist to collect prompt photons emitted from certain semiconductor materials in order to greatly improve the timing capability. In this work, we studied the prompt photon emission property and fast timing capability of two new perovskite semiconductor materials, CsPbCl3 and CsPbBr3, and compared them with TlBr, which has been previously studied for its Cerenkov emission property. The coincidence time resolution (CTR) acquired between a 3 × 3 × 3 mm3 semiconductor sample crystal and a 3 × 3 × 3 mm3 reference LYSO crystal are 238 ± 16 ps, 468 ± 55 ps and 384 ± 31 ps for CsPbCl3, CsPbBr3, and TlBr respectively, when the LYSO crystal was only triggered on photoelectric events. When we lowered the trigger level and allowed the LYSO crystal to be triggered on all events including Compton scattering, we acquired CTR of 281 ± 17ps, 499 ± 77ps and 422 ± 46ps for CsPbCt3, CsPbBr3, and TlBr respectively. Combined with the easily scalable crystal growth process, relatively low cost, low toxicity, and high energy resolution (around 3.8% at 662keV), we conclude that CsPbCl3 and CsPbBr3 could be exceptional next generation candidates as semiconductor PET detector materials.