The old show_ads did lots of work: loading additional scripts, gathering information about the web page it was running on, and building the ad request to send back to Google. The new show_ads has a different job. It creates a friendly (same-origin) iframe on the web page, and starts the old script with a new name, show_ads_impl, running inside that iframe. The _impl does all the heavy lifting, and in the end the ads look exactly the same. But there’s a substantial speed advantage: many things happening inside an iframe don’t block the web browser’s other work.
How much of an effect this has depends on context: a page with nothing but ads on it isn’t going to get any faster. But on the real-world sites we tested, the latency overhead from our ads is basically gone. Page load times with the new asynchronous AdSense implementation are statistically indistinguishable from load times for the same pages with no ads at all.
The new show_ads is a drop-in replacement for the old one: web site owners don’t need to do anything to get this speed-up. But these dynamically-populated friendly iframes are finicky beasts. For now, we’re only using this technique on Chrome, Firefox, and Internet Explorer 8, with more to come once we’re sure that it plays well with other browsers.
And what if you’ve built a page that loads AdSense ads and then manipulates them in exotic ways not compatible with friendly iframes? (This is the web, after all, land of “What do you mean that’s ‘not supported’? I tried it, and it worked!”) You can set “google_enable_async = false” for any individual ad slot to revert to the old blocking behavior. But if your site loads ads in some tortuous way because you were looking for latency benefits, consider giving the straightforward invocation of show_ads.js a whirl. Because now, we’re fast.