CAn you try a wayland session instead of X? Last Plasma update (5.6) enabled Wayland support (EGLStreams) for nVidia proprietary. You might see different behavior with MC running in XWayland.
Wow, Fedora works with wayland on NVIDIA too now. I so missed that! Gotta try that today!
I don't really have massive delay problems (although everything he said I also have, but not nearly as bad). Mostly, when I move the Window is hinks after the cursor a lot and laggs around the screen. On my notebook with an intel igpu its flawless (on wayland). Maybe the laggs would increase if my desktop had a worse GPU with xorg.
Could try xorg on my notebook and see what happens too. Yeah, when I switch to xorg on my notebook its also a mess... It is night and day better on wayland!