“Previous versions of Chrome would download a script in full before starting to parse it, which is a clear approach however doesn’t completely use the CPU while waiting for the download to finish.”
“Starting in version 41, Chrome parses async and deferred scripts on a separate thread when the download has started. This implies that parsing can finish simply milliseconds after the download has completed, and results in pages loading as much as 10% faster. It’s especially effective on large scripts and slow network connections,” Google noted in blog entry declaring the speed update.
As indicated by Google, the new technique helps to save 40% of compilation time helping page load faster. On cell phones, the new technique can likewise help cut down battery use.
The gimmick is accessible in the most recent form – Chrome 41.