Fully loaded time is measured as the time from when the visitor initially navigated to the page until 2 seconds after there’s no network activity. This event fires when a page has completely stopped loading content, including below the fold elements. This means that while your site may have loaded fast above the fold, the performance scan waits for the entire page to stop loading data before it stops, so the page load time might be longer.
Each of the three testing tools offers different options for when you want to complete a performance analysis:
Pingdom: Onload time is the only option available.
GTmetrix: Fully loaded time by default, with onload time optional.
WebPagetest: Fully loaded time by default, with onload time (aka Document Complete time) optional.