What's actually slow? A practical guide to Rails performance
SMRTR summary
A Rails development team built an observability tool and discovered that defining "slow" performance requires looking beyond averages, which can hide problems affecting real users. They recommend using P95 percentiles as the key metric—where 95% of requests perform faster than the threshold—because it catches issues affecting enough users to matter without getting overwhelmed by edge cases. They suggest flagging actions over 500ms, database queries over 100ms, and API calls over 200ms as slow, though server response time is just one component of overall user experience.
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