Is there already a performance advantage with the current WASM implementations for number crunching code compared to writing plain Javascript? My experience is that modern JS engines are already pretty good at optimizing that kind of code, so I'm wondering if there are still significant speedups to be had by using WASM, given that it's still pretty new and didn't have much time to get optimized further yet.
I may be out of date here, but IME neither JavaScript nor WASM is any good for actual "number crunching" (i.e. statistical or scientific code). For that, you want at least a compiler/language that uses the latest SIMD instructions and lets you choose 32- or 64-bit floats. Better still, you want one that automatically parallelizes loops (you may need to align your data), gives you cheap GPU access, and above all is simple enough that you can figure out why the compiler isn't performing the optimizations you expect.
Modern JS engines are pretty good at speeding up the weirdness that is JS, so maybe WASM will only have small benefits, but using either for numerical work would be making your life unnecessarily hard.