TTI marks the point when the page is visually complete and the main thread is free enough to respond to user input within 50ms. Before TTI, your page might look perfectly loaded — headline visible, CTA rendered, images in place — but clicking the CTA does nothing because JavaScript is still executing in the background. This "looks done but isn't" state is one of the most frustrating user experiences on the web.
Google Lighthouse measures TTI and considers it a significant factor in the performance score. The gap between FCP (first paint) and TTI is particularly telling: if FCP is 1.5s but TTI is 6s, visitors see content quickly but then wait 4.5 seconds for the page to become usable. During that gap, every click is either ignored or queued — and queued clicks often produce unexpected behavior when they finally execute.
Closing the FCP-TTI gap
The gap is almost always caused by JavaScript. Third-party scripts are the biggest offenders — analytics, A/B testing tools, chat widgets, and social embeds all execute on the main thread. Each one adds to Total Blocking Time, which directly delays TTI. Audit your scripts: defer everything non-critical, remove scripts that aren't providing value proportional to their performance cost, and use the Coverage tab in DevTools to find unused JavaScript.
For landing pages, ask a harsh question: do you really need all this JavaScript? A well-crafted landing page with minimal interactivity can achieve TTI equal to FCP by using server-rendered HTML, minimal CSS, and near-zero JavaScript. The best-performing landing pages I've analyzed aren't clever with JavaScript — they simply use less of it.