dual screen: some operations are slow (nouveau driver)

[Vincent Lefevre <vincent@vinc17.net>]

Hi,

I have a laptop with a 3200x1800 screen, and I now have a 4K external
monitor (3840x2160). I'm currently using the following configuration
(which is the default one, at least with lightdm):

  xrandr --output DP-3 --auto --output eDP-1 --auto --right-of DP-3

The problem is that some particular operations are now slow:

  * Moving a window is very slow, except when it is small.
    My window manager is fvwm.

  * Moving N lines at a time in a terminal is slow for large terminal
    windows. I've done a test with Mutt and half-screen scrolling,
    and I could reach a factor 3 compared to the case where the
    laptop screen is off (xrandr --output eDP-1 --off).
    I've also done a test with "less" by typing "10d" and letting
    the "d" pressed to scroll 10 lines at a time; still a factor 3,
    both with xterm and GNOME Terminal.

GNOME Terminal is slower than xterm, but the ratio between 2 screens
and 1 screen remains the same under similar conditions.

I use Debian/unstable with the nouveau driver and the 5.6.0-1-amd64
kernel. Switching to the stable kernel does not seem to change
anything.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)