The Xbox runs on a custom OS derived from Windows Core. Not the same as a consumer version of Windows.
[Edit] The answer you’re probably looking for is I/O. The PS5 is much faster than the Series X in terms of getting stuff off disk and actually using it. That more than compensates for the small speed advantage the Series X has.
I consider that the core of Windows (the NT kernel and win32 api) is actually a very polished gem but it is encased in layers of upon layers of barely polished turds ( winui, the win11 shell, the over agressive telemetry, forced ms635 integration, etc..)
[Edit] The answer you’re probably looking for is I/O. The PS5 is much faster than the Series X in terms of getting stuff off disk and actually using it. That more than compensates for the small speed advantage the Series X has.