Web browsers are getting really good at being general-purpose application platforms.
I can see the web developers drooling already.
But even though performance lags behind reasonable native environments, the divide is rapidly shrinking. Many applications have reached or exceeded the saturation of useful performance at the same time as browser developers narrow the gap between native performance and themselves.
According to David Herman of Mozilla, one of the lead authors of the ASM.js draft, the specification also allows for multithreading through web workers. Applications can take advantage of multiple hardware threads in this way, and potentially other methods as they continue development. I would expect this is especially relevant for mobile devices which tend to have relatively many cores considering their single threaded performance.
Check it out, imagine what you could be doing in your web browser in the near future.