A-Synch Ad Tags // October 6, 2013
Here’s a great article detailing the problem with synchronous ad loading. Looking at this description of how ad impressions are counted It would seem this approach could have an effect on impressions in the case where hits are recorded when the loading of assets in the ads. If you optimize your page so the time to load the page is minimal the you can get the content to your users quickly as the ads load and load the assets asynchronously so users see content much faster and just a big later.
The IAB even has a document out detailing best practices in A-Sych Ad Tech but it was written in ’08 and doesn’t mention using the approach Krug’s article mentions.