[Regr:6.4->6.5] Significant performance degradation of converting UTC from/to local time

Significant performance degradation of converting UTC from/to local time has been detected since Qt 6.5.
I have tested some QDateTime functions related to UTC from/to local time conversion with Qt 6.4 and Qt 6.8 (official builds) under Windows. In particular, `QDateTime::fromSecsSinceEpoch()` (when using with local time representation, i.e. by default) demonstrates about 50 times slower performance with Qt 6.8. against Qt 6.4.3.
(I have not performed performance measurements in all affected versions of Qt, but similar symptoms that prompted me to pay attention to this problem have been noticed since Qt 6.5.)