Re: http://blog.chromium.org/2015/03/new-javascript-techniques-for-rapid.html
As mentioned in our Chromium blog post, Chrome 41 introduces support for streaming parsing of JavaScript files using the async or defer attributes. This is where the V8 parser will parse any incoming JavaScript piece-by-piece so the compiler can immediately begin compiling the AST when script loading has completed. This lets us do something useful while waiting for the page to load. Compare:
This means parsing can be removed from the critical path when loading up the page. In these cases such scripts are parsed on a separate thread as soon as the download begins, allowing parsing to complete very soon after the download has completed (milliseconds), leading to pages (potentially) loading much faster.
