That's quite surprising. I wouldn't have imagined Windows (or any other "desktop OS") to go to great lengths to optimize for static screen content in the way that e.g. smartphones or wearables do, which as I understand have dedicated hardware optimized for displaying a fully static screen while powering down large parts of the display pipeline.
The decision to now show seconds dates back to Windows 95. Back then the motivation was not power saving, but rather to allow the code related to the clock and text rendering to be swapped out to disk on a 386 with 4MB RAM... Raymond Chen: https://devblogs.microsoft.com/oldnewthing/20031010-00/?p=42...
Desktop OSs idle most of the time, and the comparison is with respect to an idle desktop. Forcing context switches and propagating updates through the GUI stack every second isn’t free in that situation, it means that at least one CPU core can’t stay in a lower-power state. In contrast, you probably won’t see much of difference in battery life for the seconds display when simultaneously watching video or running computational tasks.
