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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: gregory@heytings.org,  56682@debbugs.gnu.org
> Date: Sat, 23 Jul 2022 11:46:37 -0400
> 
> >> AFAIK if the buffer has not been modified (including things like
> >> changing `window-start` or `point`), then a redisplay will just not
> >> run jit-lock (and hence font-lock) at all, no matter how thorough.
> > But the fact is without font-lock the response is faster by a large
> > factor.  So something, somewhere, still depends on font-lock.
> 
> Yes, that's the part that we need to explore.
> Maybe font-lock *is* run somehow?
> Or maybe it's just the mere presence of text properties?  (Or overlays?)

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.