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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  56682@debbugs.gnu.org,  dgutov@yandex.ru
> Date: Tue, 16 Aug 2022 09:07:59 -0400
> 
> > The way I see this is that Emacs is doing efforts to solve editing slowdowns
> > as best as possible, but that there is a "last mile" that cannot be solved
> > without the collaboration from major and minor modes.
> 
> The locked narrowing prevents that "last mile", sadly.

Nonsense, it doesn't prevent any such things.

Let's not exaggerate when we try to make a point, okay?