Why can two identical Windows installations behave differently on identical hardware?

If you update the BIOS between installing the two copies, the ACPI (Advanced Configuration and Power Interface) tables exposed to Windows change. This directly alters how the OS schedules threads across P-cores and E-cores (on Intel) or CCDs (on AMD).

You are comparing two Windows installations on identical hardware and seeing different behavior despite expecting equivalence. The same hardware and Windows edition are not enough: drivers, firmware, update history, optional features, power settings, startup software, and user policies can change timing and reliability. Compare the exact OS build with winver, then compare BIOS or UEFI versions, Device Manager driver provider/date/version, Windows Update history, optional updates, and power mode. Export installed drivers with pnputil /enum-drivers and compare results, especially chipset, storage, graphics, and network drivers. Next perform a clean boot on both systems and run the same test with identical user data and peripherals. If the difference disappears, re-enable services and startup items in groups to isolate it. Compare Event Viewer entries and Reliability Monitor around the test. Reinstalling immediately would erase evidence; first reduce the environments to one measurable difference and change only that item.