bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Eli Zaretskii]

> Cc: 57207@debbugs.gnu.org
> From: Ihor Radchenko <yantar92@gmail.com>
> Date: Mon, 15 Aug 2022 19:39:35 +0800
> 
> (b) Setting long-line-threshold to nil makes the slowdown disappear.
> Increasing long-line-threshold to 100000 made Emacs hang. I had to kill
> it using kill -9.

When it hangs, can you attach a debugger to it and try to figure out
where it loops?  The instructions to do so are in etc/DEBUG, under "If
the symptom of the bug is that Emacs fails to respond".  If that
procedure is more than you can afford, at least post the results of
"thread apply all bt" after attaching the debugger to a "hung" Emacs.

Thanks.