It's going to be heavily dependent on what you're doing. WASM can be as fast as native code but is more likely to be 1/2 the speed of native on realistic benchmarks. However, the overhead of moving between WASM and JS is quite large so in practical use your WASM version might be slower than a pure JS version. WASM vs JS benchmarks I've seen show WASM anywhere from 40x faster than pure JS to 10x slower than pure JS, due to this overhead.

If you're doing a large amount of CPU intensive work that can be isolated from the rest of the code that is an ideal scenario for WASM. You have the translation overhead of JS->WASM once, then do the computation, then WASM->JS once to get the result. A small function or code that needs to call back into JS (for DOM access, to query some state in your app, etc) is going to have a lot of those translations so won't see much or any benefit.

You might enjoy this thread https://www.reddit.com/r/programming/comments/7fm66l/astro_p...

This makes me wonder: does WASM allow a JIT (specializer)? I.e., does WASM allow self-modifying code or code to write code?

If not, then JIT'ed JS could still be faster than WASM.