dwm.exe high GPU usage only when duplicating display

Question

We have smart-boards at my workplace. We connect them to our new PCs with rather dated, Nvidia 710 GPUs. Everything works fine when outputting to the computer monitor, to the smart-board, or using both of them at native resolutions (1080p and 4k) in extended mode. But when we duplicate the display with the goal of mirroring what is shown on the computer's monitor, the desktop window manager (dwm.exe) starts using between 50% and 80% GPU whenever the mouse cursor moves when, otherwise, the computer is idle.

I have set the computer's power options to favour performance over energy saving, and I've done the same within the Nvidia control panel's global 3d settings, selecting max performance instead of power savings. I've scanned the PC for malware, none to be found. I've also set image scaling to [display] as the smart-board is 4k while the PC monitor is only 1080p. I've tried disabling all visual effects and animations. I installed the latest graphics drivers as they were outdated. Just updated to the most recent 2009 update. Nothing helped.

The monitor's colour scheme is RGB but the smart-board is YCBCR420, if that has something to do with anything. I can't force either display to use the same kind of colours as (I guess) our GPUs are too old, and the Nvidia control panel doesn't even show those options that the articles online shows to be there.

It's very strange that this doesn't happen while extending the displays, as it outputs both 1080p and 4k at the same time without a problem, but duplicating an 1080p image chokes the GPU.

This issue affects two similar [machine + smart-board] setups.

Does anyone have any ideas regarding this problem?

Answer 1

Haaaah... So after many hours I finally figured out how to fix this.

I don't know if this is a quirk of our old GPUs or if this is a problem with the Promethean smart-boards interacting with those GPUs, but changing the display scaling setting in the Nvidia Control Panel to GPU instead of display fixed the high GPU usage of dwm.exe.

No matter what I did, the active signal was sending a 2160p resolution despite duplicating a 1080p desktop, which I found to be strange. I thought that it might be a problem related to scaling, though I still have no idea why that would overload the GPU that much. By sheer luck I found the solution mentioned above. But as to why it acted that way I have no clue.