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

[Eli Zaretskii]

> Date: Fri, 05 Aug 2022 11:50:56 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, 56682@debbugs.gnu.org, 
> monnier@iro.umontreal.ca, 
>     dgutov@yandex.ru
> 
> Indeed, that seems to be the culprit here, I didn't know that text-mode 
> was an exception here.  If I set bidi-paragraph-direction to 
> 'left-to-right after visiting the arabic-small.txt file, Emacs (mis) 
> behaves like it does for the other Arabic files: it becomes slow, and C-n 
> C-p do not work correctly anymore.

I've installed a change that makes Arabic rendering roughly twice
faster.  It is still quite slow, but less so.