Using ffmpeg on M1 Mac is slow, hot, and the fan will kick in.
The solution is to specify videotoolbox as the encoder, which will use the hardware to accelerate the process.
Add the option
-vcodec hevc_videotoolbox for h265, and
-vcodec h264_videotoolbox for h264.
ffmpeg -i "input.mov" -vcodec hevc_videotoolbox -b:v 500k -n "output.mov"
NOTE: While much faster, the quality is not as good.