Comment by davidmurdoch

This isn't a fair comparison. Wasm was severely limited when it was first implemented and it had the advantage of a decade of improvements. Asm.js has had zero improvements in that same time frame.

Had WASM not been adopted we would have SIMD in JS ( probably via asm.js) by now. Because we didn't, JS just cannot compete with WASM in many computationally heavy workflows. We'd also have general purpose JS to Asm.js compilation, with few API restrictions, making writing it much easier.