bug#44007: 26.3; Strange shell mode performance

[Eli Zaretskii <eliz@gnu.org>]

> From: Herman@debbugs.gnu.org, Géza <geza.herman@gmail.com>
> Date: Thu, 15 Oct 2020 11:53:16 +0200
> 
> I'm experiencing strange shell (comint) mode performance. If I cat a 10 
> MB file, it takes 1:20 to cat it. Emacs uses only 30% cpu. But, if I 
> press enter after I send the cat command, it speeds up, and it only 
> takes 8 sec (emacs cpu usage goes up to 100%).
> 
> Same thing: if I execute "seq 100000" in a small emacs window, it takes 
> ~3 seconds. But, if I press enter after executing seq, it takes only 0.4 
> sec (this is not 100% reproducible, sometimes pressing enter doesn't 
> make a difference).
> (I executed comint-clear-buffer before measurements)

Can you please describe a detailed reproduction recipe, starting from
"emacs -Q"?  It is not exactly clear from the above whether you do
this in a shell-mode buffer or in some other mode, where exactly do
you press Enter, etc.  A detailed recipe will resolve all those
unclear aspects.

Thanks.