On my machine this gives me:
```
********* Start testing of tst_QTimeZone *********
Config: Using QtTest library 6.10.0, Qt 6.10.0 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 14.2.1 20250207), arch unknown
PASS : tst_QTimeZone::initTestCase()
PASS : tst_QTimeZone::utc()
RESULT : tst_QTimeZone::utc():
358.686871 nsecs per iteration (total: 358,686,513, iterations: 999999)
896.524312 CPU cycles per iteration, 2,5 GHz (total: 896,523,416, iterations: 999999)
2,227.000427 instructions per iteration, 2,484 instr/cycle (total: 2,226,998,200, iterations: 999999)
560.000375 branch instructions per iteration, 1,56 G/sec (total: 559,999,815, iterations: 999999)
PASS : tst_QTimeZone::cleanupTestCase()
Totals: 3 passed, 0 failed, 0 skipped, 0 blacklisted, 374ms
********* Finished testing of tst_QTimeZone *********
```
Profiling shows some quite unexpected code paths that
I will try to optimize in follow-up patches. Note that
this function can be called frequently when deserializing
QDateTime over a QDataStream e.g. - I have stumbled over
it while profiling some KDE PIM code in akonadi.
Pick-to: 6.5 5.15
Change-Id: I7439df53ae8512c766f63cb4b0d4f33d14aa3a01
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 29c7beac2e0065cc0d03b9db6a701f7c690c6f56)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit c4618d6020b1ab568f40338fb715142d8fe01445)
fd8b35a121