bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii]

> Date: Mon, 01 Aug 2022 12:20:39 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: dgutov@yandex.ru, 56682@debbugs.gnu.org, monnier@iro.umontreal.ca
> 
> > Having to wait for the initial display is pretty annoying.
> 
> It is, indeed.  But here we are talking about big files (say 1 GB).  In 
> that case (and in that case only) it is much better, from a user 
> viewpoint, to wait say 20 seconds before the file is opened and being at 
> that point able to freely move through the file, instead of waiting only 6 
> seconds, and then having to wait another 10 seconds between two motion 
> commands like M-> C-p.  In fact, no user expects that a 1 GB file would 
> open instantaneously.

You can maybe have that for C-p that follows M->, but wouldn't the
wait return, with a vengeance, if you insert a single character
(because then the buffer needs to be re-scanned)?  If so, we've gained
nothing, really.