It's here, here at last! Only about a decade too late, and about six or seven years after Google announced they were working on it.
AdSense now offers asynchronous code, finally. It looks something like this:
<script async src="http://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
<ins class="adsbygoogle"
style="display:inline-block;width:728px;height:90px"
data-ad-client="ca-pub-xxxxxx"
data-ad-slot="xxxxxx"></ins>
<script>
(adsbygoogle = window.adsbygoogle || []).push({});
</script>
Asynchronous code is non-page blocking, meaning it doesn't slow down the rendering of the page, and only loads the ads after the rest of the page has already finished loading.