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

[Clemens Eisserer]

Hi,

I am using KDE on an AMD Kaveri based system featuring a 4k display.
Quite a few operations/animations feel rather slow on that machine
(e.g. the semi-transparent selection window on the plasma desktop, or
the slide-animation in kicker).
So animations inside windows (drawn by QT) are slow, the compositor
related animations are fluent and fine.

Looking at the profiles it all boils down to QT using the "raster"
backend. I see tons of cycles spent inside functions like
qt_memfill32, comp_func_solid_SourceOver_sse2 and the likes. No wonder
it feels slow - performing a single operation covering the entire
screen means modifying 16mb (per frame!) and pumping those from system
RAM to the GPU.

Is there any way to permanently switch to the "opengl" QT graphics system?
I tried e.g. to create a file in /etc/profiles.d (e.g.
https://github.com/KDE/plasma-active-kickstart/blob/master/scripts/qt-graphicssystem-opengl.post)
and set:
export QT_GRAPHICSSYSTEM=runtime
export QT_DEFAULT_RUNTIME_SYSTEM=opengl

or even only

export QT_GRAPHICSSYSTEM=opengl

Yet, regardless or exporting it directly in the shell where I start
e.g. dolphinor in the config file, as soon as I play with e.g. a
selection rectangle I see those software loops in operation.

Any idea how to switch permanently to the opengl qt graphics backend?

Thank you in advance, Clemens