bug#11943: 24.1.50; Emacs unusably slow when looking at large files (bidi support at fault)

[Eli Zaretskii <eliz@gnu.org>]

> From: Dima Kogan <dima@secretsauce.net>
> Date: Sat, 14 Jul 2012 17:50:51 -0700
> 
> I'm observing that when some large text files are loaded, emacs slows to
> a crawl. As an example, I have a 14MB file open (with emacs -Q). Every
> time I do (next-line) or (previous-line) it takes a few seconds. This is
> a > 2GHz Core2 machine, so there's no reason for this to happen. 'M-x
> benchmark' says that (previous-line) takes >2s each time. I discovered
> that if I do (setq bidi-display-reordering nil) then emacs is snappy
> again, with previous-line taking <1ms.
> 
> The specific file I'm using to exhibit the bug consists of many repeated
> stanzas such as
> 
> =========================
> {
>  {2.222222,2.222222,2.222222,2.2},
>  {-2.222222,2.222222,2.222222},
>  {-22.222222,22.222222,2.222222}
> },
> =========================
> 
> without the =. Saving a stanza into a file called 'snippet', the 14MB
> file can be made with
> 
> $ for i in `seq 17`; do cat snippet snippet > xxx; mv xxx snippet; done

What is the major mode in the buffer where you see this?  I mean the
real-life example where you bumped into this, not the 'snippet' file
produced by the above.