The problem comes from too much content having to be downloaded to render the visible page on a mobile device. Ideally it'll only take one request (round trip) to display that.
I got rid of the warning on one page by moving an image down a couple lines on the page.
If you need those images on your mobile pages, you'll have to take the hit (or make them lower resolution).
If you get this error on a lot of your pages, you should rethink your layout or what you're loading in the <head> that could be loaded later (befor </body>).