Re: Why emacs rendering is slow when encountering long lines?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 20 Oct 2013 03:25:30 -0700 (PDT)
> From: Jiaxin Cao <jiaxin.cao@gmail.com>
> 
> On Sunday, October 20, 2013 1:00:45 PM UTC+8, Jiaxin Cao wrote:
> > When I open a file containing long lines, it is not smooth to scroll up and down, even if the file is not large. 
> > 
> > 
> > 
> > Does anyone know how to configure emacs to make it smoother?

Hard to tell without knowing what was in the file.  The performance is
somewhat sensitive to the contents of the file.

But in general, the Emacs display engine is known to have trouble with
very long lines, see bug #13675.

> Specifically, scroll-down is way worse than scroll-up. 
> 
> I did a little test for it. A single scroll up takes about 15ms to finish, while a scroll down takes about 50ms. 
> 
> So if you press down pageup to trigger consecutive scroll-downs, the screen will basically not respond to anything. 
> 
> Anyone can explain why?

The reasons for the asymmetry are complex and have to do with the
display engine internals.  But first let me ask you: when you say
scroll-up, do you mean go forward in the buffer, or do you mean going
back?

Also, what was the major mode of the buffer where you tried this?