Emacs on my first computer, an i486 with a whopping 33 MHz of compute power had no trouble with syntax highlighting, autocomplete and spell checking.
The problem with slowness is in the design, not inproved functionality. The root cause is that modern software runs a ton of checks for every keystroke, tries to talk to the mothership every second and is designed to advertise and upsell instead of solving actual user problems.
TBF, Emacs/vim today with pretty much every feature turned on also don't feel sluggish.
I think the 2 issues hitting modern text editors are they are FAR too synchronous (they are doing the syntax highlighting, fonts, etc, with every keystroke rather than in the background).
And rendering has gotten WAY too nuts. Seems like all modern text editors are full blown web browsers, usually so they can be easily cross platform.
The problem with slowness is in the design, not inproved functionality. The root cause is that modern software runs a ton of checks for every keystroke, tries to talk to the mothership every second and is designed to advertise and upsell instead of solving actual user problems.