1) when did they downgrade? I've stared for hours at that particular code...

2) no you just don't get to move data freely.

3) I don't believe JSON has any place in a system that needs C because it can't handle Rust.

4) JSON can't handle non-tree structures, it's further very limited in expressivity. Rkyv is more of a code gen akin to ASN.1

Happy benchmarking, feel free to use the rkyv benchmark tooling and ensure you have enough link time optimization going on.