How to use the "opengl" qt rendering backend instead of "raster"?

[René J.V. Bertin]

>Window content painting is always under control of the application -
>it doesn't make a difference whether compositing is enabled or not.

I agree it shouldn't and I've tried to defend that in a discussion with a KWin developer a while back. He seemed to argue otherwise. It's true that KWin is more than a window manager alone.

>Whenever I cause a redraw of a KDE-5 application, I always see a lot
>of cycles spent in software rendering/blending loops - even for
>operations which can be easily done with OpenGL (blits with alpha).

If in pure, generic Qt code there's only so much they can do to optimise drawing across the range of all supported platforms, without introducing overhead (and while preserving the possibility for X11 users to use a remote display).

>The kde page "KDE Mailing Lists" only shows this list - and a separate
>list for developers.
>Is there a separate list for plasma questions?

I don't think there is, but your question is technical enough to pose on a developers' list even if you're doing any Plasma development yourself.
This particular list is mostly driven by users; I answered because experience leads me to expect you will probably not get much more helpful feedback here.
But you'd do good to figure out what those software rendering loops are actually drawing.

R.