Time to Interactive
AdvancedTime to Interactive (TTI) is the moment when the page is not just visible but actually responds to the user's clicks and typing without delay.
The gap between “looking done” and “working” is exactly what TTI captures: a page that appears fully loaded but whose main thread is still busy running heavy JavaScript will not answer the user's touch and feels broken. TTI is a lab metric that marks the moment when, after FCP, the main thread stays quiet for five seconds with no heavy work queued; in Lighthouse scoring, roughly under 3.8 s is good. Importantly, TTI is not suited to measuring responsiveness from real users and, in the field, has given way to INP, which directly measures the response to each real interaction. So read TTI as a lab warning about heavy JavaScript, and turn to INP for real responsiveness.
How it is calculated
TTI = the first moment after FCP when the browser's main thread stays free of heavy work for five seconds
The output is a time, and computing it requires tracing the main thread's long tasks, so it is a lab measure; for field responsiveness, INP is its replacement.