cpu-used is (somewhat confusingly) not for how many threads it uses, but how good the quality of the video should be in a scale from 0 to 9, where 0 is best quality and slowest, and 9 is worst quality and fastest. Very similar to how -preset was used for x264.

https://github.com/AOMediaCodec/community/wiki#how-to-make-e... explains the different settings that most change the speed of encoding.