I've ported a template compiler to Rust before. The reality is you still end up with something slower than native JS because of the overhead serializing and parsing the JSON in WASM memory. This is something that will be helped by the interface types proposal[0] if it is implemented but I was surprised that this use case didn't work as well as I thought it would.
[0] https://github.com/WebAssembly/interface-types/blob/master/p...