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

[Eli Zaretskii]

> Date: Fri, 5 Aug 2022 23:23:25 +0300
> Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> > We need to look at more than just M->.  C-n/C-p, C-v/M-v, C-l are also
> > important, as are the time it takes from typing M-x or M-: until you
> > see the prompt in the minibuffer, and the time to update the display
> > after inserting or deleting a single character.
> 
> I'm not seeing any particular sluggishness in these operations when 
> visiting dictionary.json.

Numbers, please.  You have a very fast machine, so what doesn't look
sluggish on your system could very well be so on others.

> > Thanks, so I guess we may have a solution for JSON files, if disabling
> > syntax-propertize-function doesn't have any downsides.  What about
> > other modes that we see in files with long lines, like XML?
> 
> Someone will need to test it with some typical large file. xml-mode 
> (alias to nxml-mode) does have a syntax-propertize-function, but it's 
> probably faster than js-syntax-propertize.
> 
> > And how scalable is the solution you propose, i.e. how it behaves in
> > JSON files with a much longer lines?
> 
> parse-partial-sexp is O(length of text span)

Isn't that the part you proposed to remove?  I was asking about the
remaining parts.

> Meaning, it scales linearly. You'll see a 10x delay in a JSON file that 
> is 10x as large.

Linear scaling is less optimal than O(0), which is what the current
solution produces.