How should a Python service hash passwords, tune the work factor, and upgrade old hashes?

Hash cost is a capacity decision as well as a cryptographic one. In an async service I keep CPU-heavy verification off the event-loop thread or use a bounded worker path, then measure tail latency and saturation under login abuse. Rotation supports multiple recognized hash formats during migration but creates only the current format. Metrics count algorithm versions and rehash outcomes without labels or logs that expose user identifiers or hash material.