"The fold" is the imaginary line that separates the part of the page which is visible without scrolling, and the rest of the page. Lots of web developer effort is targeted at that initially visible part of the page. The concept is, of course, stupid, but today's web developers are incapable of making fast web sites without the help of half a dozen "frameworks", which is to say they are incapable of making fast web sites. This creates a need to "optimize" for something, because an overall good result is right out. I heartily recommend this keynote speech by Maciej Cegowski about the website obesity crisis: https://player.vimeo.com/video/147806338

I have bad news for you: If you think that Google's servers are overloaded, then your ISP is slow/throttling. I can download Youtube videos at 100 Mbit/s any time of the day, any day of the week.

Google's webmaster tools speed measurement gave my web site a "yellow" score even though Google's own main page, google.com, loaded slower than my page, which has lots more content. If Google tells other people that my web site slow, it can expect a response to that defamation. No, Google, I'm not going to add Javascript to lazily load images below the fold. My site loads faster than yours for a reason.