Google claims to have made the show_ads.js script faster making billions of webpages showing adsense load those ads faster. Adsense is a factor in slow loading of some pages, around 12% of the page load time.
The page should load half a second faster.
Adsense publishers don’t have to upload new code since the change is automatic. Right now, the changes are working with Chrome, Firefox, and Internet Explorer 8, with more browsers and versions included soon.
If you need to block the new ad version, set “google_enable_async = false” for each ad slot.
Full post from Google Blog Click here