bug#57245: 29.0.50; M-> in a large XML file (without long lines) is slow

[Dmitry Gutov]

On 16.08.2022 23:22, Stefan Monnier via Bug reports for GNU Emacs, the 
Swiss army knife of text editors wrote:
> Dmitry Gutov [2022-08-16 22:32:23] wrote:
> > On 16.08.2022 19:54, Eli Zaretskii wrote:
> > > Stefan, can you see why syntax-related stuff in sgml-mode is so heavy
> > > here?
> > nxml-syntax-propertize might well be heavier than average, but the delay
> > scales linearly with the size of the file.
> Indeed, it should be linear.
>
> > Which seems to be exactly the behavior the "font-lock narrowing" was
> > supposed to guard from?
> Not sure which narrowing you're referring to.
> The "locked narrowing" introduced by Gregory is only installed in the
> presence of long lines.  It's (currently) not used for large files
> (unless they contain long lines, that is).

I guess that's the problem here.

The font-lock narrowing (if it's indeed the method we're going to use to 
speed up its performance) shouldn't be conditioned on the presence of 
long lines.