One of the bigger problems with emacs is its font highlighting mechanism - typically it's implemented with regular expressions adding stacked attributes to text. Lexical highlighting using a state machine, with state recorded at line start would be much much faster - like, multiple 1000x faster. Emacs pays for its flexibility with massive performance loss.

Compare something like joe editor (the jmacs named executable has emacs-like key bindings) for syntax highlighting; it's night and day. Joe uses state machines (DFA) in a text description format for syntax highlighting. That means that strictly it can only do lexical highlighting, but that's much much better than nothing - and fully correct syntax highlighting requires a deeper analysis than is normally done in emacs modes.

Joe itself would be much faster again if the state machine was compiled to machine code.