According to a code comment in QLocale::uiLanguages(), the function has O(N³) complexity. OTOH, the only use of QTranslatorPrivate::load_translation()'s QLocale argument is a call to uiLanguages(), so we might as well just pass the QStringList result of that function call to load_translations() instead of the QLocale itself.. Do that, and then call uiLanguages() before taking the QTranslator mutex. There are still tons of IO operations performed under the lock, so this is drop on the hot stone, but it's also easy to do, so do it. Amends the start of the public history. Task-number: QTBUG-138851 Pick-to: 6.10 6.9 6.8 6.5 Change-Id: Ieea899dd2829437e659f7b22dfcf578efe35ed00 Reviewed-by: Ivan Solovev <ivan.solovev@qt.io> Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>