bug#44464: 28.0.50; slow shell output

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 44464@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>
> From: Phil Sainty <psainty@orcon.net.nz>
> Date: Fri, 6 Nov 2020 02:54:17 +1300
> 
> I can reproduce the issue (in 27.1), and it seems to me that it's
> all about that one paragraph of right-to-left text.
> 
> If I disable `bidi-display-reordering', I see no slow-downs.

I don't see any particularly bad slowdown at all, but maybe I didn't
make all the steps (I don't have 'curl').  Do you see the same
slowdown if you fetch the URL with, say, WGET, and then visit it in
Emacs?  If not, how to reproduce this without 'curl'?

> Eli, I note that setting `bidi-paragraph-direction' in the buffer
> (with bidi-display-reordering enabled) does not seem to have any
> obvious effect.

It normally doesn't.  Only some very special files can benefit from
setting bidi-paragraph-direction non-nil.

> Setting it to `left-to-right', I still see right-to-left movement
> through that portion of the text, and the associated slowness.  Is
> that expected?

Yes.  Paragraph direction doesn't disable reordering, it just
determines whether the text will start at the left or the right edge
of the window.

> I'll add that the line in question is not *especially* long
> (2374 characters); however it does seem to be a significant factor,
> as enabling longlines-mode eliminates the slowness around that text.

I might be able to say something intelligent about this if I have a
recipe for reproducing the slowdown, and have some quantitative idea
of the slowdown (how much time does it take for Emacs to respond to
some specific command).

Thanks.