ffmpeg - Conversions - Convert H264 to AV1 - libaom - Constant Quality

libaom-av1 has a constant quality (CQ) mode (like CRF in x264 and x265) which will ensure that every frame gets the number of bits it deserves to achieve a certain (perceptual) quality level, rather than encoding each frame to meet a bit rate target.

This results in better overall quality.
If you do not need to achieve a fixed target file size, this should be your method of choice.

time ffmpeg -i a.mp4 -c:v libaom-av1 -crf 30 -f mp4 out.mp4

returns:

...
real	260m18.018s
user	1514m33.124s
sys	0m43.892s

NOTE:

Input test size: 75.5M
Output test size: 24.5M

NOTE:

crf: This value can be from 0–63.
- Lower values mean better quality and greater file size.
- 0 means lossless.
- A CRF value of 23 yields a quality level corresponding to CRF 19 for x264 (source), which would be considered visually lossless.