In this article https://www.hackster.io/eugene-tkachenko/comparing-video-stream-latencies-raspberry-pi-5-camera-v3-6c0ee4 author compared different approaches and found that using

picam-vid -t 0 --camera 0 --nopreview --codec yuv420 --width 1280 --height 720 --inline --listen -o - | ffmpeg -f rawvideo -pix_fmt yuv420p -s:v 1280x720 -i /dev/stdin -c:v libx264 -preset ultrafast -tune zerolatency -f rtsp rtsp://localhost:$RTSP_PORT/$MTX_PATH

will be most efficient.