Re: Slow redisplay with long lines

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Thu, 15 May 2014 16:38:52 -0400
> 
> > Long lines slow down redisplay because it needs to scan the entire
> > line to see how tall it will be on display.
> 
> Why should it care about the whole line rather than only the part
> of it that's displayed?

It does indeed care only about the displayed part of the line.  At
least it tries to, although sometimes it is hard.  E.g., when you need
to go back, you need to start from the beginning of the previous line,
because only there you know you are at column zero.

>         Stefan "who presumes there are several other reasons why long
>                 lines cause severe performance issues"

The need to traverse the long lines is the single most import one,
because it comes into play at the lowest level of display iteration,
and so pops up in many higher-level operations.