like 6+ months later...finally got around to testing around with virtual syntax highlighting with Prism in <light-editor>
experiments seem slowdowns seem to start at ~10k+ lines, up from ~1k-2k lines
A lot of the slow down is calculation with no throttling on every keystroke, tokenizing fully, etc
Theres quite a bit of low hanging fruit and bugs to fix as well as layout thrashing...but as a whole...cool to get it working
and a little visualization with the <pre> overlaying the <textarea>