This study successfully develops a novel high-performance detector using rare-earth-doped cesium lead chloride (CsPbCl₃) inorganic scintillation crystals, targeting the critical requirement for GHz-rate capabilities in ultrafast radiation detection at advanced light sources. The Ba²⁺-doped CsPbCl₃ crystals, grown by the vertical Bridgman method, exhibit sub-nanosecond fluorescence rise times, with the measured average rise time from 10% to 90% of pure crystal being ~209.6 ps and that of optimized doped crystals achieving ~50—75 ps. The crystals also exhibits nanosecond-scale decay times and enhanced light yield, achieved through defect engineering. By integrating this core scintillator with a microchannel plate photomultiplier tube (MCP-PMT) that features sub-nanosecond transit time and a self-developed 2.5 GHz high-speed acquisition system, a complete ultrafast detection system is constructed. Rigorous testing with the use of an optically generated equivalent GHz pulse train demonstrates that the system can clearly resolve consecutive fluorescence pulses with an average peak interval of only 0.79 ns, successfully achieving a high-repetition-rate detection capability of 1.26 GHz. The field application at the Shanghai soft X-ray free-electron laser (SXFEL) facility demonstrates that its X-ray pulse response width is narrower than 4 ns, far superior to the >24 ns response of a reference LYSO:Ce crystal. These results validate the detector's exceptional sub-nanosecond time resolution and GHz-rate pulse discrimination, providing a reliable technical solution for ultrafast time-resolved diagnostics and photon beam loss monitoring in next-generation scientific facilities. The following figure shows a novel rare-earth-doped CsPbCl₃ perovskite ultrafast scintillator detector and its achieving sub-nanosecond time resolution and GHz-rate pulse discrimination (1.26 GHz).