[NV108] Quadro K610M: Some operations are slow with dual head

On my HP ZBook 15 G2 under Debian/unstable (5.6.0-1-amd64 kernel package, corresponding to 5.6.7) with GK208GLM [Quadro K610M] (rev a1), GK208 (NV108) chipset, some operations become slow when I connect and enable a 4K external monitor (3840×2160). In particular:

• 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.

Note that this performance issue occurs whether I use a side-by-side configuration (7040×2160 frame buffer, where 7040 = 3840 [4K external monitor] + 3200 [laptop screen]) with

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

or screens at the same position with

xrandr --output DP-3 --auto --output eDP-1 --auto --same-as DP-3

i.e. with the same content on both screens (except that the laptop screen, which has a lower definition, does not show some bottom and right parts). In this latter case, the frame buffer is set to 3840×2160, corresponding to the 4K monitor. But if I disable the laptop screen (with xrandr --output eDP-1 --off), while the frame buffer has not changed, the performance issue disappears.

So the slowness seems to be caused only by the fact that two outputs (monitors) are enabled, not by the frame buffer size.

The NVIDIA proprietary driver does not have this issue.