The author is experiencing a battery drain issue on their Apple Watch whenever the macOS system process called secd is using a high percentage of the CPU on their Macs. The problem arises randomly, and the only solution is to delete a specific folder from the Keychains on both Macs, followed by a complex process of signing back into iCloud and re-authenticating accounts. Despite seeking help from Apple Support and scouring the web, no permanent fix has been found. The author resorts to periodically unpairing and setting up the Apple Watch from scratch to resolve the issue temporarily.
https://arstechnica.com/civis/threads/secd-chews-up-cpu-cycles-and-watch-battery-life-goes-in-the-terlet.1485088/