It's how the combination of the three encourages you to do things if you want to have a site with rich content. You're supposed to paint the page ASAP, so you d

It's how the combination of the three encourages you to do things if you want to have a site with rich content. You're supposed to paint the page ASAP, so you don't want to defer loading any large content, but then you're not supposed to have the page layout shift around at all as you dynamically load all that content later, so you've got to do clever things with placeholders and swapping out content and whatnot. You've got up to 4 seconds to load all that stuff, which is enough to load an enormous amount of data over a fast internet connection, so much so that the same amount of content might take minutes to load over a slower connection. Fortunately, they've chosen methods for measuring that metric that are heavily biased toward measuring the experience of people who have 24/7 access to broadband. So, yeah, Google may want to encourage a nice Web experience, but they don't want to back this with metrics that might discourage people from sending too much business in AdSense's direction, or fail to favor Chrome over alternative browsers.