Mobile operating systems (OS) are frequently updated, but such updates can unintentionally degrade user experience by introducing performance regressions. Existing detection techniques often rely on system-level metrics (e.g., CPU or memory usage) or focus on specific OS components, which may miss regressions actually perceived by users -- such as slower responses or UI stutters. To address this gap, we present MobileUPReg, a black-box framework for detecting user-perceived performance regressions across OS versions. MobileUPReg runs the same apps under different OS versions and compares user-perceived performance metrics -- response time, finish time, launch time, and dropped frames -- to identify regressions that are truly perceptible to users. In a large-scale study, MobileUPReg achieves high accuracy in extracting user-perceived metrics and detects user-perceived regressions with 0.96 precision, 0.91 recall, and 0.93 F1-score -- significantly outperforming a statistical baseline using the Wilcoxon rank-sum test and Cliff's Delta. MobileUPReg has been deployed in an industrial CI pipeline, where it analyzes thousands of screencasts across hundreds of apps daily and has uncovered regressions missed by traditional tools. These results demonstrate that MobileUPReg enables accurate, scalable, and perceptually aligned regression detection for mobile OS validation.