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.