I run a forum that shows a single Google AdSense unit on thread pages. I ran a page that includes a Google ad via the WebPageTest, and I was horrified to find out that Google, who is all for web page optimization, is slowing down my page by 9 HTTP requests! That’s 7 javascripts and 2 .png files!
Isn’t there something I can do about this?
Yeah, that’s why you often hear “ads suck” 
I know the adsense team said they were working on improving the performance (and making it load async) but it’s been a while since they talked about it (velocity last year). I expect that if they don’t have an announcement or update this year then there’s going to be a lot of questions and pressure (rightfully so since they are responsible for so much of the ads on the web).
The alternatives aren’t really pretty. I’ve seen a bunch of people try to make it load async but there were issues with all of the solutions. You could switch to an ad network that supports async loading but they might not be able to monetize as well. You could remove the ads but that kind of defeats the purpose
Here is a way to use DFP (formerly google ad manager) to load ad slots asyncronously. DFP has a great adsense fallback option, so you can use it if you want to run both adsense-only or adsense mixed with other networks/advertisers.
This method is a hack and may break. It may violate TOS.
Thanks Pat and Sajal. I looked at the above linked solution, and I’m not sure how I would implement it. I will have to read more to understand it.