Rendering of Japanese text slower than latin text

This issue results from https://bugreports.qt.io/browse/QTBUG-129586 

Its original description describes two issues and provides a reproducer.

One of the issues is the difference in performance when resizing the window when the english text is on screen versus when the japanese text is visible.

After some measurement, it seems the difference comes from the fact that in void Renderer::prepareAlphaBatches() there is a difference in bounding boxes.

When comparing the english and the japanese version, the english version never enters bool Renderer::checkOverlap(int first, int last, const Rect &bounds) which the comments suggest we want to avoid calling. The japanese version ends there quite often.