After upgrading to Hermes v0.17.0, the Mac's idle power draw jumped from a normal 3-5W to 19W and the fans spun up. Using powermetrics to quantify, ps to localize the process, and sample to grab the call stack, I traced it all the way to Chromium's Rust font engine fontations and the Rust implementation of the Temporal API, temporal_rs, repeatedly formatting time strings and validating glyphs every frame, pinning the GPU at full load. This post records the full investigation.