The image below shows where asynchronous compilation doesn't stop the main thread from executing, allowing the browser (and whatever JS-powered application is running at the time) to continue unabated. In this case the app is the Mandreel test from the Octane benchmark:
According to Guo, the Nexus 5 saw a 27 per cent performance increase in the Mandreel test, thanks to this development and other browser-based application should run better across the board.
Note that concurrent compilation isn't available in the stable version of Chrome, you'll have to grab the beta to test it out.
Compiling in the background for a smoother user experience [Chromium blog, via TheNextWeb]