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

[Dmitry Gutov]

On 13.08.2022 09:13, Eli Zaretskii wrote:
> > > I can only repeat that the only ways to improve font locking in a non-local
> > > way (or at least, the only ways that were identified in the discussions so
> > > far) are either to improve the speed of syntax-ppss by an order of
> > > magnitude, in which case it would perhaps become feasible to allow
> > > syntax-ppss to have access to the whole buffer, or to equip syntax-ppss with
> > > heuristics to make it work "as well as possible" even when it only has
> > > access to a portion of the buffer.
> > But the locked narrowing is applied based on the presence of long-lines
> > rather than based on the total size of the buffer.
> That's true, but the two are closely related: a buffer with very long
> lines tends also to be a large buffer.

The long-line related problems start to become noticeable much earlier 
than the buffer becomes "large".