bug#44007: 26.3; Strange shell mode performance

[Eli Zaretskii]

> Date: Sun, 5 Nov 2023 17:46:15 +0100
> From: Herman, Géza <geza.herman@gmail.com>
> Cc: 44007@debbugs.gnu.org
> 
> I think I found another thing which is needed: tty echo must be enabled. 
> With bash, echo is disabled by default, so the issue doesn't happen. 
> With zsh, echo is enabled by default, and the issue happens (I'm not 
> sure why zsh turns on echo).
> 
> So either one has to use zsh to repro the issue, or use these updated steps:
> 1. emacs -Q
> 2. M-x shell RET
> 3. stty echo
> 4. seq 100000
> 
> With echo disabled, "seq 100000" takes ~0.2 sec. With echo enabled, it 
> takes 3 sec (and pressing RET immediately after executing seq makes it 
> fast).

I'm not sure what exactly are we trying to do here.  AFAIU, at least 2
variables were discovered that can speed up this example, so what else
has to be done?  Is there any bug here, and if so, what is the bug?