Time to Interactive(TTI)
Time to Interactive (TTI) is the lab metric for when a page becomes reliably responsive — first FCP, then 5 seconds of network and main-thread quiet. Removed from Lighthouse's scoring weight in v10 (2023) and effectively deprecated as a public metric. INP replaced it for measuring real-user interactivity.
Long definition
TTI was Google's headline interactivity metric for years: the moment a page is "ready for the user", defined as the point after FCP when the main thread has been quiet for at least 5 seconds with no long tasks (>50ms) and no more than 2 in-flight network requests. The intent was to capture not just visual readiness but functional readiness — a page that looks loaded but freezes on click is not interactive.
The metric had two structural problems. First, the 5-second-quiet definition is a lab construct that doesn't map cleanly to real user behavior — users don't wait 5 seconds; they click immediately. Second, TTI was extremely sensitive to small changes in network or main-thread quiet windows, making it noisy in field measurement and hard to act on.
Google's response was to keep TTI as a diagnostic in Lighthouse lab reports while removing it from the performance score weight in v10 (2023). For field interactivity, INP — Interaction to Next Paint — became the Core Web Vital in March 2024, replacing First Input Delay. INP measures actual user interactions, not a synthetic readiness threshold.
If you still see TTI in audits, treat it as advisory: a slow TTI usually indicates heavy JavaScript execution after FCP, which is the same problem TBT measures more reliably. Optimize for TBT in lab and INP in field; TTI tracks them as a side effect.
Common misconceptions
- "TTI is still a Core Web Vital." It never was. Core Web Vitals are LCP, CLS, and INP (previously FID). TTI was a Lighthouse lab metric.
- "TTI is the same as INP." They measure different things. TTI is a single lab readiness threshold — when did the page get quiet enough to consider interactive. INP is the worst-case (98th percentile) latency of real user interactions across the session.
- "A good TTI guarantees good INP." Not directly. TTI says the page reached a quiet state; INP says individual interactions are fast. A page can pass TTI and still have one expensive click handler that blows out INP later in the session.
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