Thank you.
After going through that linked guide once again, I realized that I had disabled 3D acceleration. Enabling it improved the overall situation (no more graphical glitches), but performance was still awful - simple window dragging was like slow motion. Fortunately, switching to QXL, as you suggested, solved the problem completely 
What still puzzles me though is that when researching this topic, very often virtio is claimed to offer the best possible performance and is recommended as the default choice, yet for some reason in my setup with this light DE this is not the case 