Time to Interactive (TTI) is a lab metric that measures when a page is interactive. It’s calculated by first marking the page’s FCP. From there, the next five-second period in which there is neither a long task nor more than two in-flight network requests is also marked. The page’s TTI is at the start of that quiet window. Sites with a high TTI will behave more like screenshots of a website than an actual website, making users think your website is busted. A good TTI is less than five seconds on a middle-tier device (http://bkaprt.com/rjs39/03-07), but at the risk of repeating myself, I am once again asking you to aim as low as possible.