bug#15555: 24.3; Bidirectional display very slow with long lines

[Eli Zaretskii]

> Cc: 15555@debbugs.gnu.org
> From: Jerome L Quinn <jlquinn@us.ibm.com>
> Date: Tue, 8 Oct 2013 11:39:58 -0400
> 
> I'm not sure it is the same bug.  When I disable the bidi reordering
> variable,
> navigation speed becomes reasonable, even with the very long line length.
> I'm on a very fast machine, so #13675 may not be impacting me that badly.
> 
> When bidi reordering is enabled, it is multiple seconds to move the
> cursor up and down, which is unusable.

You are on a very fast machine, so you don't see the slow redisplay
with such lines.  But the basic problem remains: Emacs does not cope
well with long lines.  It's just that in your case the border of
"unbearable" is farther.

> Setting bidi-paragraph-direction to right-to-left improves the
> situation some but I'd still call it unusable in this situation.
> Disabling reordering makes emacs as responsive as I'd expect,
> comparable to emacs23.

Emacs 24 cannot possibly work as fast as Emacs 23, since reordering of
bidirectional text does need a lot of additional processing.  There's
nothing that can be done about that, without a complete rewrite of the
Emacs display engine (or purchasing a faster machine).

> I don't see as bad behavior when the text is in the left buffer or
> if I have only 1 window.

Then don't do that.

Again, these all are signs of slow redisplay with long lines.  They
just become a bot faster or slower in specific situations and screen
arrangements.

> And disabling bidi reordering completely eliminates the bad behavior.

If you can afford that, go for it.