Some additional information... When I change the configuration to
xrandr --output DP-3 --auto --output eDP-1 --auto --right-of DP-3 (usual dual screen settings), /var/log/Xorg.0.log shows: modeset(0): Allocate new frame buffer 7040x2160 stride because 3840 + 3200 = 7040 and max(2160,1800) = 2160. And when I do xrandr --output eDP-1 --off to disable the laptop screen, /var/log/Xorg.0.log shows: modeset(0): Allocate new frame buffer 3840x2160 stride corresponding to the external 4K monitor. I was wondering whether the performance issue with the dual screen settings could be related to the size of the frame buffer. So I tried xrandr --output DP-3 --auto --output eDP-1 --auto --same-as DP-3 in which case, the frame buffer size is still 3840x2160, and both screens show the same content (except that the laptop screen, which has a lower definition, does not show some bottom and right parts). But even in this case, I notice the same performance issue. I would add that if I switch to a Linux console (Ctrl-Alt-F1), I get the same contents on both screens (though only 3200x1800 is used), and everything is very fast: I can't "move windows" there, but the scrolling tests do not show any performance issue, unlike with xorg. Any idea? -- Vincent Lefèvre <vinc...@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)