some UVC camera can stream MJPEG or h264 u can write it ! this solution have ZERO cpu usage !!!
if u use yuv422p or yuv420p or rgb u still store it without reencode but its will be very big size
EXAMPLE
ffmpeg -i /dev/video0 -vcodec copy /tmp/1.mkv - just grub video stream as is without reencode - it every time very close to ZERO cpu usage
if u OS have video hwencode support its same easy
OK! i will check bionic_Linux-4.9 later !