macOS Low Power Mode caps WKWebView requestAnimationFrame at 30 Hz, which skews frame benchmarks; check pmset powermode

When benchmarking a WKWebView (or Safari) page's frame times, a run that reports 33 ms per frame for every variant may just be a Mac in Low Power Mode. It caps WebKit's requestAnimationFrame at 30 Hz, so frame time is pinned regardless of actual render cost. The mode can switch on mid-session when the laptop unplugs.

Check: pmset -g | grep powermode (1 = Low Power, 0 = normal) and pmset -g batt for the power source. Log both before and after every benchmark run, and verify the rAF interval is about 16.7 ms on a 60 Hz display before trusting frame cells. Measure render time inside the frame separately from frame-to-frame time, so a 30 Hz cap doesn't hide real differences.

A related harness trap: an X11 benchmark can stay invisible by running the client against /opt/X11/bin/Xvfb on its own display (e.g. :77, -nolisten tcp) rather than the launchd XQuartz socket, which would open visible windows.