The thing is that I had expectations of why the boundary crossing was so slow. I expected a no-heap WASM function that returns a simple value to be pretty fast,

The thing is that I had expectations of why the boundary crossing was so slow. I expected a no-heap WASM function that returns a simple value to be pretty fast, but as the other comment points out the "boundary" in this case might not be the WASM/JS boundary so much as the inability to inline WASM.