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

[Stephen Berman]

On Wed, 10 Aug 2022 13:11:52 +0000 Gregory Heytings <gregory@heytings.org> 
wrote:

>>> I'm not completely sure he did indeed remove those three brackets. Stephen,
>>> can you remove the three characters at positions 1, 22 and 23, and try
>>> again? These are the first two '{' and the first '[' characters in the
>>> buffer.
>>
>> I did remove those brackets previously; the improvement I see is that,
>> without them (and even without removing the corresponding closing brackets
>> at the end of the file, and of course with bidi-inhibit-bpa nil), holding
>> down `C-n' moves the cursor almost a whole screenful (~60 visual lines)
>> before coming to a halt, instead of just three visual lines with the initial
>> brackets, and after letting go of `C-n', the cursor catches up to the actual
>> position of point noticeably quicker than with the unaltered file.  Still,
>> the movement is not at all smooth, unlike with bidi-inhibit-bpa non-nil (and
>> that is not as smooth as moving through the same file with so-long-mode
>> enabled).
>>
>
> Is that with emacs -Q? And with show-paren-mode disabled?

I had forgotten to disable show-paren-mode; doing that (with -Q) seems
to increase the length of the initial scrolling but it still comes to a
halt and there's a delay (though no longer several seconds) before the
cursor catches up.  Further scrolling in the file seems not to show the
same improvement (i.e., scrolling halts again after ~3 visual lines),
though the file is too short to do that more than once or twice.

Steve Berman