Comment by nine_k
Why, running KDE in VirtualBox in full-screen mode must be fine :) At least, I did it breathlessly with Xfce, on much older Apple hardware, and it was... just fine.
(OTOH running text-mode Emacs from a headless VM in a full-screen built-in Terminal may suddenly feel sluggish. Kitty or WezTerm solves this.)
Last time I tried, it didn't work well (or at all) with multiple monitors.