ffmpeg v4l2 taking 100% cpu usage

Question

I am on Fedora 37, version is irrelevant as I had same issue on Fedora 36.

I am using the following command to send video from my usb video card to v4l2loopback and the CPU usage goes to 100% and I can't do much more on my computer (i5-8600k, nvidia 2060 rtx, 16GB ram).

ffmpeg -nostdin -threads 1 -f video4linux2 -video_size 1920x1080 -input_format mjpeg -i /dev/video1 -f v4l2 -preset fast -c:v copy /dev/video0

previously the command was like that and the rest of the arguments was added to see if it makes any difference, it seems it doesn't change much

ffmpeg -f video4linux2 -video_size 1920x1080 -input_format mjpeg -i /dev/video1 -f v4l2 -c copy /dev/video0

To be honest I don't know much about ffmpeg or video in general, I was using the same command a few months ago on the same hardware with almost 0 impact to my performance, I could play games and watch videos at the same time while now I get 15 FPS in my games while I am using ffmpeg. Did anything change in ffmpeg? Any improvements to my ffmpeg command? Shouldn't ffmpeg not encode anything if I am using pixelformat and frame rate supported by my usb video card and if that is true shouldn't be super light?