Javascript JITs are really good, you (likely) aren't going to see major performance improvements by dropping into WASM.

That said, one major benefit of WASM that Javascript jits will have a hard time competing with is GC pressure. So long as your WASM lib focuses on stack allocations, it'll be real tough for a Javascript native algorithm doing the same thing to compete (particularly if there's a bunch of object/state management).

For a hot math loop doing floating point calcs (mandelbrot calc), however, I've seen javascript end up with identical performance compared to WASM. It's really pretty nuts.