On: Emacs Modernization: Simple Changes Emacs Should Adopt (2006)

[jpeloquin]

> some of them weird Unicode characters, so not just ASCII

This makes me suspicious that it's the font cache causing the slowdown (assuming Windows), not long lines. I saw similar symptoms a couple years ago and fixed it with (setq inhibit-compacting-font-caches t).

You could also try increasing the garbage collection threshold on the theory that the lag is a gc pause. I've used:

  (setq gc-cons-threshold (* 511 1024 1024))
  (setq gc-cons-percentage 0.5)
  (run-with-idle-timer 5 t #'garbage-collect)