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^^^And THAT RIGHT THERE is why I load the asynchronous Adsense script at the very bottom of my pages, after everything else, and why I load my core JavaScript synchronously, just before that, so it can schedule key tasks before getting swamped with third party guff.

I mean, sure, Adsense loads asynchronously and Google claims it won't slow your page loads, but once it starts loading advertising assets and scripts it's the Wild West. Some - not all, by any means, but some - of those third party scripts are extremely badly behaved and, yes, they do measurably slow your page load down.