The way lazy load plugins work, in layman’s terms, is by keeping the major assets a ‘secret’ until the client needs them. When the server sends its initial response, the HTML that the client receives has placeholders instead of a reference to an asset. This way, the client isn’t waiting to receive these assets before rendering the page for the user.
As such, Google will also be clocking the Page Load Time as though the site is already complete. This not only means that the user doesn’t wait too long to see the site but Google assumes that the site loads much faster than without the app.
As the user scrolls down the page, the missing assets are transferred as needed from the server, wthout the user having to waste any extra time waiting.