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

[Eli Zaretskii]

> Date: Mon, 1 Aug 2022 04:23:21 +0300
> Cc: 56682@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>,
>  Stefan Monnier <monnier@iro.umontreal.ca>
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> IIUC this state of affairs is caused by your chosen approach to speeding 
> up font-lock (hard narrowing while it is called), which makes the 
> initial call to syntax-ppss happen inside that narrowing as well.
> 
> The alternative being that font-lock would call syntax-ppss right away 
> with no restriction, but then only apply highlighting to limited parts 
> of the buffer.

AFAIU, this seems to assume that highlighting is much faster than
syntax-ppss.  Is that a given?  If not, I don't think I understand how
this could help.

>  > It is way better to wait
>  > a few seconds more while the file is being opened than to wait before
>  > two basic motion commands when the file is already opened.
> 
> I agree, yes.

I don't.

> > It doesn't only solve the syntax-ppss problem, it also makes 
> > flyspell-mode usable in such files, for example.
> 
> Does flyspell-mode always scan the full buffer?

flyspell-mode doesn't scan anything but the text you type, as you type
it.  Anything else must be explicitly invoked by the user.