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

[Gregory Heytings]

> > > > The general idea is that syntax-ppss is currently an order of 
> > > > magnitude too slow for "too large" buffers.
> > >
> > > And that it can be almost another order of magnitude worse in some 
> > > major modes because of `syntax-propertize`.
> >
> > That's possible, I don't know.  I might take a look at that later. 
> > But note that your reactions are not exactly encouraging me to do so.
>
> I don't see any need to look into it, so don't waste your time there,

To make syntax-ppss faster, if possible?  Is that not a sensible thing to 
do?