A couple of things here. WASM isn't necessarily more performant than vanilla JS. For the web, it'll open the door to the JS engine as a compile target, and that's great.... but it's still not super fast.

Secondly, in the case of an application, WASM is of little use since (with node and almost any programming language) you have an FFI available. One could simply shell out computationally heavy things to C++ or whatever else. This is what many folks do with their Electron apps already (via node-ffi).