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

[Eli Zaretskii]

> Date: Tue, 02 Aug 2022 14:57:25 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: Eli Zaretskii <eliz@gnu.org>, 56682@debbugs.gnu.org, 
>     monnier@iro.umontreal.ca
> 
> > Regarding the long-standing bug reports, we did solve a bunch of issues 
> > already. One major one, IIUC, was redisplay of already fontified text on 
> > long lines.
> 
> Try to open the dictionary.json with Emacs on master a month ago.

I believe it can also be done with the current master, just after
setting long-line-threshold to the nil value.  Right?