From the site: The kind of binary format being considered for WebAssembly can be natively decoded much faster than JavaScript can be parsed (experiments show more than 20× faster). On mobile, large compiled codes can easily take 20–40 seconds just to parse, so native decoding (especially when combined with other techniques like streaming for better-than-gzip compression) is critical to providing a good cold-load user experience. – from the FAQ of WebAssembly

If you build a website and have to parse something for 20-40 secs for a user, they should fire you on the spot for bad design. Start working for adult sites, if loads longer then 2 secs, redesign.