(In reply to comment #12)
> I noticed something weird: if I start KDE with desktop effects off it is
> *much* faster and there is corruption in the title bar.

The corruption indicates it's using the native Qt backend, not Raster.

> Also I noticed it is much faster when I set resolution to 1920x1200 compared
> to 2560x1600, at least when starting KDE with desktop effects off.

2560x1600 is almost twice as many pixels as 1920x1200, so some difference in
performance between them is expected.