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

[Gregory Heytings]

> > Especially given that the said correctness has always been relative.  
> > So it's not like we're leaving the heaven of 100% correctness and 
> > falling into the hell of 10% correctness.  A more reasonable view of 
> > the situation is that we had 90% correctness and now have,
>
> As someone who worked on different major mode and syntax-ppss itself a 
> little, that feels moderately insulting.
>
> No: we strive to close to 100% correctness in supporting language syntax 
> and can often reach it with moderate effort (programming-wise).

It isn't meant to be insulting, and I think you understood that.  My 
experience is simply that highlighting in Emacs is definitely not "close 
to 100% correct", whatever be the mode.  The most correct one is perhaps 
emacs-lisp-mode (unsurprisingy).

> > in files with "loo long lines" and only in those files, 60% 
> > correctness.
>
> Does the attached screenshot look like 60% correctness to you?

A small sample of a big file is definitely not representative, and I think 
you understood that.

> To me, it's more like -60%. Or at least, that's what the utility of such 
> highlighting will be (negative).

In which case you should turn highlighting off in such files.  That's what 
all other editors do anyway.