It's doing copies and (de)serialization on both sides into native data types.

If they used raw byte structures, implemented the caching improvements on the wasm side, the copies might not be as bad.

But they still have an issue with multi-language stack: complexity also has a cost.

Python/C combo does not have this issue because you can work with Python types natively in C, but otherwise, this is a cross-language conversion issue, and not a Rust issue at all.