bug#40007: 28.0.50; Emacs gets very slow when displaying a long line

[Eli Zaretskii]

merge 40007 13675
thanks

> From: Jan Synacek <jsynacek@redhat.com>
> Date: Tue, 10 Mar 2020 10:48:24 +0100
> 
> 
> 1) emacs -Q
> 2) M-x shell
> 3) execute a program that displays a single line, about ~193000
> characters long
> 
> I accidentaly wrote a scheme program that dumped an entire file as a
> single line, about 193k characters long, into the shell buffer. Emacs'
> reponse latency becomes *very* slow, especially when navigating the
> point over the long line.

This is bug#13675 (and others that were merged with it).

We now have so-long-mode to alleviate some of the problems.