[kam]

No, serde-wasm-bindgen implements the serde Serializer interface by calling into JS to directly construct the JS objects on the JS heap without an intermediate serialization/deserialization. You pay the cost of one or more FFI calls for every object though.

https://docs.rs/serde-wasm-bindgen/

[szmarczak]

Indeed, you're right. However, it still needs to encode and decode strings. WASM just needs native interop.