[Performance regression 5.6->5.15->6.7]QWidget-based QScrollArea drops across versions on Boot2Qt device

Reproducer attached. Way to reproduce: run a Wayland compositor first, e.g. minimal-qml, and then the reproducer with -platform wayland-egl. The reproducer has a built-in FPS counter on top-left corner. Have only tested on Apalis iMX6 and release build.

It is said that the FPS was about 40~50 if the reproducer is built with Qt5.6. But it is ~20 if it is Qt5.15. Qt6.7 seems to be worse: only about 10~15 FPS.

It is the situation with wayland-egl. Default EGLFS is 1~2 FPS across versions and thus not very much useable at all.

The QML-based scrolling is much better but original reporter is not moving from QWidget anytime soon.