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

[Eli Zaretskii]

> Date: Sat, 23 Jul 2022 19:05:54 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: Stefan Monnier <monnier@iro.umontreal.ca>, bug-gnu-emacs@gnu.org
> 
> > My bet is indeed on the mere presence of text properties, plus the fact 
> > that we need to merge faces.
> 
> That's precisely what I've been investigating during the last day.  I 
> probably need two or three more days to reach a conclusion, so please tell 
> me if you're doing that in parallel.

I'm not.  I did the measurements, and so did Gerd, but I'm not looking
into the font-lock influence any deeper than that.