bug#10835: 24.0.93; bidi-paragraph-direction slows down Shell mode

[Chong Yidong]

Chong Yidong <cyd@gnu.org> writes:

> I just noticed that when I do `ls --color=no' rather than just `ls',
> insertion and scrolling are very fast.  The sluggishness only appears
> when ls color highlighting is enabled.  So the inefficiency of
> ansi-color.el, and its use of overlays, seems to play a role.

I just confirmed that the sluggishness goes away if I change
ansi-color.el to make it use text properties.  Wish I'd remembered to do
that before the pretest began.

Roughly what is happening, I think, is that when overlays are heavily
used and there are no paragraph breaks (e.g. from the `time' output),
the bidi code does enough the additional overlay lookups to produce a
noticeable slowdown.

I'm not sure what's the best fix right now, though.