In order to create more efficient encodes when a particular target bitrate should be reached, two-pass encoding should be used.

Two-pass encoding is also beneficial for encoding efficiency when constant quality is used without a target bitrate.

• For two-pass, you need to run ffmpeg twice, with almost the same settings, except for:
  • In pass 1 and 2, use the -pass 1 and -pass 2 options, respectively.
  • In pass 1, output to a null file descriptor, not an actual file. (This will generate a logfile that ffmpeg needs for the second pass.)
  • In pass 1, you can leave audio out by specifying -an.

time ffmpeg -i a.mp4 -c:v libaom-av1 -b:v 2M -pass 1 -an -f null /dev/null && time ffmpeg -i a.mp4 -c:v libaom-av1 -b:v 2M -pass 2 -c:a libopus -f mp4 out.mp4
 
time ffmpeg -i a.mp4 -c:v libaom-av1 -b:v 2000k -pass 1 -an -f null /dev/null && time ffmpeg -i a.mp4 -c:v libaom-av1 -b:v 2000k -pass 2 -ac 2 -c:a libopus  -f mp4 out.mp4x

returns:

...
real	98m40.037s
user	567m32.622s
sys	0m26.253s