Here's an interesting study over at the TinyTube.net blog regarding the performance of ad networks on smartphones and PDAs:

As it turns out, ad pulls from various mobile ad networks account for a significant fraction of the page load time... I wanted to get some hard numbers on how much each ad network is slowing us down. So we collected data for each network, sample size is 100... Looks like Admob is a clear winner for speed. Decktrade and Millennial are clear winners for consistency, everyone else gives a > 1s response at some point...

Through this post, the TinyTube.net blog has highlighted a consideration (Ad Network Quality of Service) for those looking to create mobile-ready websites and monetize them. The connectivity sluggishness caused by ad networks may slow your visitors mobile browsing experience significantly, turning them off and precluding future visits.

Thanks to the operators of TinyTube.net for the heads-up via email.