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A thread at WebmasterWorld named Does AdSense cause a Download Lag? asks for confirmation on that question. What he gets are methods to increase the perceived load time of the AdSense advertisements.
I use tables and the browser displays the page real snappy and then you see AdSense fill in as it can.
If you position your layers through the stylesheet then you can load them into the browser in the order of content and load the ads layer last.
Pack your ad units into a div, and give that div a fixed size in pixels (the same size as the ad unit, of course). This allows the browser to position that div and to immediately continue with the rest of the layout.
Other suggestions in the thread.
