>I want to know what conceivable workflow in Inkscape would allow those sorts of figures in even a significant fraction of lecture speed, let alone real-time

https://github.com/gillescastel/inkscape-shortcut-manager

https://castel.dev/post/lecture-notes-2/

I've also done LaTeX for notes (in EMACS), but was never happy with how things looked when doing everything quick. The sheer genius of the setup he made is eye-opening.