The newly-emerging ‘fail-slow’ failures plague both software and hardware where the victim components are still functioning yet with degraded performance. To address this problem, this paper presents Perseus, a practical fail-slow detection framework for storage devices. Perseus leverages a light regression-based model to fast pinpoint and analyze fail-slow failures at the granularity of drives. Within a 10-month close monitoring on 248K drives, Perseus managed to find 304 fail-slow cases. Isolating them can reduce the (node-level) 99.99th tail latency by 48.05%. We assemble a large-scale fail-slow dataset (including 41K normal drives and 315 verified fail-slow drives) from our production traces, based on which we provide root cause analysis on fail-slow drives covering a variety of ill-implemented scheduling, hardware defects, and environmental factors. We commit to releasing the dataset to the public for fail-slow study.