Microsoft explains why Windows 11 doesn’t get seconds on the taskbar
Another controversial issue is that the time module in the taskbar no longer displays seconds, which upsets many users who like to display seconds in Windows. In fact, Windows 10 does not support displaying seconds by default, but users can manually turn it on by modifying the registry so that the hours, minutes, and seconds can be displayed.
Microsoft writes:
On multi-users systems, like Terminal Server servers, it’s not one taskbar clock that would update once a second. Rather, each user that signs in has their own taskbar clock, that would need to update every second. So once a second, a hundred stacks would get paged in so that a hundred taskbar clocks can repaint. This is generally not a great thing, since it basically means that the system is spending all of its CPU updating clocks.