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

[Gregory Heytings]

> > > I think we could have both speed and correctness, at least for files 
> > > of this size.
> >
> > Feel free to enlarge long-line-threshold in your init file.
>
> Just a curious question: How did we arrive at the current value of 
> `long-line-threshold'?

I took the same value that Stefan had chosen for syntax-wholeline-max.

> I've been trying to follow the discussion but I don't think this has 
> been mentioned anywhere.  My apologies if I missed it.

No need to apologize, you're a careful reader 😃, it hasn't been mentioned 
indeed.