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

[Gregory Heytings]

> > It is necessary to add a locked narrowing around 
> > fontification-functions and pre/post-command-hook to ensure that Emacs 
> > remains responsive.
>
> Perhaps tweaking the value of syntax-wholeline-max could have a similar 
> enough effect?

It doesn't, alas.

> > Unless I misunderstand something, I think (and my tests seem to 
> > confirm) that that would be a workable solution, provided that the 
> > initial scan is reasonably fast, that is, at least six times faster 
> > than it is now.
>
> I'm afraid this approach is too blunt to be a solution.

Indeed, that's my conclusion too.  So until syntax-ppss (at least) is made 
an order of magnitude faster, the right thing to do is to use the forced 
narrowing method.