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

[Dmitry Antipov <dmantipov@yandex.ru>]

On 02/18/2014 09:42 PM, Eli Zaretskii wrote:

> Anyway, just moving cursor horizontally cannot possibly be slow due to
> bidi, especially as long as point stays in the same screenful.  The
> redisplay becomes unbearably slow with long lines only when you either
> scroll the display (e.g., C-v) or for vertical cursor motion, because
> these require the display engine to traverse many buffer positions,
> many more than is needed to just move the cursor, and it currently can
> only start that traversal from the beginning of a physical line.

1) I realize that vertical motion is slower than horizontal, but [2] from
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15555#65 shows that the major
slowdown happens when cursor is moved horizontally (by right-char) within
the same line.

2) (setq bidi-display-reordering nil) helps bug15555 to run over 4000.txt
just as expected.

Dmitry