I have 4 monitors with mixed refresh rates running on KDE and used to have high GPU usage plus high usage on one cpu core. The settings I found that seemed to fix it were Allow Flipping in nvidia-settings, and using Option “UseNvKmsCompositionPipeline” “false” in xorg conf. Turning off Allow Flipping got rid of the high cpu usage, and the xorg option got rid of the high gpu usage. Now on idle desktop, cpu is at 0% and gpu is at 0-2%. May or may not help in your case, but maybe worth a try.