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

[Eli Zaretskii]

> Cc: 56682@debbugs.gnu.org, gregory@heytings.org
> Date: Sat, 23 Jul 2022 19:15:26 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > 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.

Btw, I think the best tool for determining this is run-time profiling,
such as with perf on GNU/Linux.