bug#44464: 28.0.50; slow shell output

[Eli Zaretskii]

> From: "Basil L. Contovounesios" <contovob@tcd.ie>
> Cc: Phil Sainty <psainty@orcon.net.nz>,  andreas.roehler@easy-emacs.de,
>   44464@debbugs.gnu.org
> Date: Thu, 05 Nov 2020 14:41:07 +0000
> 
> 0. emacs -Q
> 1. M-x shell RET
> 2. C-x 1
> 3. wget -O- http://dbpedia.org/resource/Earth
> 4. RET

Thanks.  Looks like a regression between Emacs 26.3 and 27.1.  And
it's somehow related to auto-composition-mode, because turning that
off makes Emacs fast again.

Bisections welcome.

> While printing, the last line is also highlighted in blue, i.e. with the
> comint-highlight-prompt face.

I think this is part normal, as the last portion of display in shell
mode is always highlighted like that.