You're right, QPC is not known to be very stable, I should have checked that first.
It would be great if someone having this problem could comment the contents of the Platform::GetSystemTime function in src\SFML\System\Win32\Platform.cpp, just keeping this line
return GetTickCount() * 0.001;
(I know it's not timeGetTime, but this will be enough for tests)
Then recompile SFML and test again
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