Re: C-n is very slow in Font-Lock mode

["Eli Zaretskii" <eliz@gnu.org>]

> From: Richard Stallman <rms@gnu.org>
> CC: eliz@gnu.org, emacs-devel@gnu.org
> Date: Tue, 26 Apr 2005 18:56:25 -0400
> 
>     One way to do that is to set jit-lock-defer-time.
> 
> Are you suggesting this as a change in the defaults?
> I'm not interested in this as a workaround for me alone;
> I want to make it faster for everyone.

It could be the default (a value such as 0.2 would be good, I think),
although I'm not sure how users will react: the current behavior in
21.x series of Emacs is effectively as if jit-lock-defer-time were
nil, and I don't think I've seen a significant number of complaints
about scrolling.

Personally, I'd reset that option to nil if its default were changed,
because I rarely page through a buffer with C-n, and because on modern
machines the slowdown is barely visible, but the flash of unfontified
text is annoyingly obvious.  But that's just me.

> I found a way to change line-move-1 not to use vertical-motion
> in simple cases.  I think it should work, but I don't understand
> the reason why the code uses vertical-motion.

It's ancient history, from before the display engine was rewritten for
Emacs 21.  Here's the relevant entry from the logs:

  1995-03-09  Richard Stallman  <rms@pogo.gnu.ai.mit.edu>

	  * simple.el (line-move-ignore-invisible): New variable.
	  (line-move): If that var is set, use vertical-motion.
	  Skip any extra invis chars beyond where vertical-motion stops.

So it seems like you ought to know the reason ;-)