I understood it as compression is 25% better : a quality of 10mbps in av1 can be achieved with 8mbps in Av2.
But, it needs 5 times more compute power for this 25% gain.
dav1d is the av1 decoder and it’s an insane feat of engineering. Written in assembly, it even eschews the normal c calling convention to get even better performance.
The normal C calling convention is really only for cross-binary calls (e.g. between shared libraries). If you're not doing that you can ignore it; it's not a weird thing to do. It would be odd to strictly follow it in assembly and I assume compilers don't either.