Problem A: Plasma is sluggish and unresponsive. This can be remedied (somewhat) by making it use the raster graphics engine.

So 'export QT_GRAPHICSSYSTEM=raster' right?

Problem B: On recent KDE versions, the raster engine makes KWin much slower.

So, edit /usr/share/autostart/plasma-desktop.desktop to include '-graphicssystem=raster', right?

Problem C: That works, but it will be overwritten every time the KDE packages are updated!

How can I make Plasma, and only Plasma, use the raster graphics system, without having to reapply the tweak every time KDE is updated? Can I e.g. put the edited version of plasma-desktop.desktop in my local KDE dir?

If you create a file ~/.config/autostart/plasma-desktop.desktop containing a "[Desktop Entry]" and "Exec" entry, then it should override the system-wide entry.

Okay, thank you very much!