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

[Eli Zaretskii]

> Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca,
>  visuweshm@gmail.com
> Date: Sat, 23 Jul 2022 20:49:39 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > > Font-lock Scroll-bar Composition Output
> > > ------------------------------------------------------------
> > > on          on         on          GCs: 16 Elapsed time: 5.496764 seconds
> > > on          off        on          GCs: 1 Elapsed time: 5.362916 seconds
> > > on          off        off         GCs: 1 Elapsed time: 3.947306 seconds
> > > 
> > > That's with
> > > 280b8c96cc origin/feature/long-lines-and-font-locking Improve display of 
> > > columns on mode-lin
> > 
> > Thanks.  So the scroll-bar effect is largely gone, and font-lock is
> > now just 25% slower than no-font-lock.  Which I think is reasonable,
> > given that there's a face change every 10 to 20 characters?
> 
> Oops, I was confused: font-lock OFF is not in the table, so with it
> we're still 4 to 5 times slower than without it.  It's
> auto-composition mode that costs us 25% slowdown.

Which seems to be similar to slowdown due to font-lock in other cases?
For example, scrolling with the same benchmark through xdisp.c takes
190 sec for the first time and 40 for the second time (when everything
is already fontified); whereas without font-lock it takes 20.  So it
sounds like font-lock generally slows down redisplay by such small
factors?