If it's just for benchmarks, why not make it a preference (defaulted off), so benchmarks can be run with the default settings (off) just fine, but people can turn it on if they want to?
1) People with it on would say "Windows is slow".
2) Because it adds code to maintain that doesn't need to be.
Displaying seconds is a valuable feature, so it's not a matter of having unnecessary code. As for being slow, the ability to display seconds was removed because of the minimum system requirements for Windows. Anyone with a machine that has more than the bare minimum can probably spare the kilobytes necessary to draw seconds.
Certainly more useful/less code than the analog clock in the date/time settings window.