bug#44464: 28.0.50; slow shell output

[Phil Sainty]

On 5/11/20 11:55 pm, Basil L. Contovounesios wrote:
> Andreas Röhler <andreas.roehler@easy-emacs.de> writes:
>> When running
>> curl -L -H "Accept: text/html" http://dbpedia.org/resource/Earth
>> output stops at several times.
> 
> Isn't this just because of long lines in the output?
> This is a well-known performance pitfall in Emacs.

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.

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

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.


-Phil