Overview
TTI identifies the point at which a page has displayed useful content (FCP), event listeners are registered for most visible elements, and the page responds to user interactions within 50 milliseconds.
Why it Matters
A page might look ready (visual load) but be 'frozen' because the main thread is busy executing heavy JavaScript. TTI helps identify this 'uncanny valley' where users try to interact with a page that isn't yet responsive.
Optimization
Improving TTI usually involves reducing JavaScript payload sizes, splitting code into smaller chunks, and deferring non-critical scripts.