Capturing video from the rpi camera with ffmpeg can vary from less than 5% to 100% of the CPU (rpi zero) depending on ffmpeg using the hardware acceleration or not.
On many github issues one finds the suggestion of using h264_omx
codec to use the gpu - but it does not ship with the default ffmpeg
on Raspbian.
Instead I found that one can use the v4l2 driver provided by raspbian to get hardware accelerated h264 output. Also setting the video size will save one from using a (cpu) scale filter.
capture h264 video from rpi camera