The QDate.format function is not very efficient when you have to format a large number of dates, because every time it is called it always has to parse the string that contains the format.  
Java has the DateTimeFormatter class that allows the format string to be set once . The instance then is passed to the Date::format function.
The date format is generally always the same, so adding a QDateTimeFormatter class would easily improve the performance of applications that must format many dates.

See: https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html