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

[Gregory Heytings]

> > Given your and Dmitry's feedback, I just tried to add an explicit call 
> > to (syntax-ppss (point-max)), without narrowing, when the buffer is 
> > opened (see below).
> >
> > The problem is that this is, as I said, slow.  On my laptop, opening a 
> > 1 GB file takes about 6 seconds.  The call to syntax-ppss adds 70 
> > seconds, so opening a large file becomes an order of magnitude slower 
> > (13 times slower).  Which I think is too much for the added benefit.
>
> But that only has to happen when the buffer is scrolled to the bottom, 
> right?

No, it happens when the buffer is opened.  Given the importance that you 
and Stefan seem to give to that function, it is, with the patch I sent in 
my previous post, called once on the whole buffer (without any narrowing) 
when the file is opened.  Later calls (inside fontification-functions or 
post-command-hook) are subject to a forced narrowing.

> And syntax-ppss's speed depends on the rules applied by the particular 
> major mode. Those could be sped up. Some optimization of this function's 
> speed is not out of the question either.

They would be more than welcome.  In fact, without such optimizations, it 
would be unreasonable to do what the patch does.