Hi this is as a sample of adsense code I use and that already contains an async instruction within the script tag:
<style>
.example_responsive_1{width:300px;height:250px}
@media (min-width:500px) {.example_responsive_1 {width:336px;height:280px}}
@media (min-width:800px) {.example_responsive_1 {width:728px;height:90px}}
</style>
<!-- 4icu.org-Level3_Bottom -->
<ins class="adsbygoogle example_responsive_1"
style="display:inline-block;min-width:250px;max-width:1140px;width:100%;height:100%"
data-ad-client="ca-pub-MYADSENSEID"
data-ad-format="rectangle"></ins>
<script>(adsbygoogle=window.adsbygoogle||[]).push({});</script>
However async still downloads the ads included in a page no matter whether they are located in the page if I am not wrong. So if I have three adsense ad slots in the same page async will defer the ad load but not avoid the heavy load itself like lazy loading would do.
If mod pagespeed would include a lazy load option that prevent the bottom ads to be downloaded before they are scrolled down and visible it would save so much total page size for those visitors that do not bother scrolling till the end of the page.
So more than 1 MB of total page size and 3 seconds of page load time are just due to the two Adsense slots and the Google Custom Search.
Any mod-pagespeed that would lazy load the adsense ad slots placed at the below the initial fold view would help limiting the page size and increase the page load speed considerably.
If a specific option for lazy loading adsense ad slots was included in the mod-pagespeed it would be very beneficial for all publishers monetizing through Adsense.
Cheers