toString() in macOS' system locale is very slow

My app, that needs to filter a big table model, is very slow on macOS. Upon investigation, I found out that all QLocale().toString(int/double/QDateTime) are 2-3 orders of magnitude slower than the same calls on QLocale::c().

Attached is a simple benchmark tool that produces the output below. You'll see that we're currently around 100x slower in release builds (debug builds are around 1000x slower, but a big part of that is the QMacAutoReleasePool in the macOS query() callback).

Here is a patch on gerrit that adds caching to any QSystemLocale call that returns a constant string: https://codereview.qt-project.org/c/qt/qtbase/+/446822

Int and enum returns from query() are currently not cached and dynamic data (e.g. QDateTime as parameter to query) cannot be cached at all anyway.

baseline, dev branch, release build on MacBook Air M2

       Locale: C     10000x toString(3.14): 0s 003ms 069us
       Locale: de_DE 10000x toString(3,14): 0s 003ms 020us
System Locale: en_DE 10000x toString(3,14): 0s 408ms 052us
       Locale: C     10000x toString(12345): 0s 000ms 331us
       Locale: de_DE 10000x toString(12.345): 0s 000ms 704us
System Locale: en_DE 10000x toString(12.345): 0s 382ms 285us
       Locale: C     10000x toString(Monday, 5 December 2022 04:05:29 CET): 0s 032ms 768us
       Locale: de_DE 10000x toString(Montag, 5. Dezember 2022 04:05:29 CET): 0s 032ms 779us
System Locale: en_DE 10000x toString(5. December 2022 04:05:29 CET): 2s 457ms 831us

with caching patch:

       Locale: C     10000x toString(3.14): 0s 003ms 243us
       Locale: de_DE 10000x toString(3,14): 0s 003ms 346us
System Locale: en_DE 10000x toString(3,14): 0s 005ms 205us
       Locale: C     10000x toString(12345):  0s 000ms 926us
      Locale: de_DE 10000x toString(12.345): 0s 001ms 809us
System Locale: en_DE 10000x toString(12.345): 0s 002ms 975us
       Locale: C     10000x toString(Monday, 5 December 2022 04:02:32 CET): 0s 047ms 544us
       Locale: de_DE 10000x toString(Montag, 5. Dezember 2022 04:02:32 CET): 0s 032ms 981us
System Locale: en_DE 10000x toString(5. December 2022 04:02:32 CET): 0s 035ms 714us