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

[Eli Zaretskii]

> Date: Sun, 24 Jul 2022 17:35:19 +0300
> Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> On 24.07.2022 08:50, Gerd Möllmann wrote:
> > Eli Zaretskii<eliz@gnu.org>  writes:
> > 
> >>> My bet is indeed on the mere presence of text properties, plus the
> >>> fact that we need to merge faces.  But I could well be wrong.
> > Can't say something about face merging, but "frequent" changes of faces
> > certainly have an effect on iterator performance.  It stops, looks up
> > properties again to determine the next stop pos, does what has to be
> > done for current properties...
> 
> But the problem is contingent on having long lines, isn't it?

Not necessarily, see the times I measured scrolling through xdisp.c,
which I posted earlier.  It could be that with long lines font-lock
just makes it slower still, to the point where it becomes unbearable.

> There must be some interplay between those circumstances. Not just 
> having to look up faces (relatively) a lot.

What else did you have in mind?