Wasm is smaller and faster than JavaScript and asm.js. It is both faster on average, it has far better predictability (far fewer performance cliffs or pitfalls)

Wasm is smaller and faster than JavaScript and asm.js. It is both faster on average, it has far better predictability (far fewer performance cliffs or pitfalls), and it starts up faster (faster to decode + no need to warm up the JIT with types at runtime). You can test this pretty easily, since Emscripten still supports JavaScript output with a flag (for environments that lack wasm support for whatever reason). Comparing Emscripten's default wasm output to JS output for the same benchmark will show those benefits in most cases.