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

[Gregory Heytings]

> > > FWIW, I strongly object to the addition of such a variable (unless it 
> > > is clearly stated that it is added temporarily and should not be used 
> > > except for testing purposes).
> > >
> > > Adding such a variable only two weeks after locked narrowing has been 
> > > introduced means that modes will have little incentive to adapt to 
> > > that stronger constraint, if they can "fix" whatever problems that 
> > > constraint might cause by setting that variable to nil in their 
> > > initialization hooks.
> >
> > I don't think major modes would take advantage of that var. I hope not, 
> > at least.
>
> I wouldn't mind because if things get slow, we get to point the finger 
> squarely at the culprit.

If it is clearly stated that it should only be used for testing/debugging 
purposes, yes.  Otherwise it's just another variable that anyone can use 
as they see fit.