Again, slow is relative, and JS is slow relative to WASM. That should be obvious, but here's a source.[0] A demo working smoothly does not mean that a language is not slow compared to another. Besides this point, there are some very processing heavy tasks that we're throwing in browsers these days; this demo is not representative of everything we expect browsers to run. And if you read the other comments here, some people were having lag issues with the demo anyway.
[0]: https://hacks.mozilla.org/2017/03/why-webassembly-is-faster-...