Time to Interactive was removed from the Lighthouse report in version 10 (February 2023) and no longer contributes to the Lighthouse performance score. For responsiveness, use Total Blocking Time in the lab and Interaction to Next Paint in the field. Calibre keeps Time to Interactive for historical reference only.
Time to Interactive (TTI) was a lab metric that measured how long a page took to become reliably interactive: visually rendered, with event handlers registered, and able to respond to input quickly. It was a useful idea, but in practice it was fragile, and the metrics that replaced it describe responsiveness more accurately.
What is Time to Interactive?#
Time to Interactive reported the moment a page became reliably able to respond to user input. It was a lab metric, measured in a controlled synthetic test rather than from real users, and it depended heavily on main-thread activity.
How was Time to Interactive calculated?#
Time to Interactive was derived from main-thread and network activity after First Contentful Paint:
- Start at First Contentful Paint.
- Search forward for a five second window that is quiet: no long tasks, and no more than two in-flight network GET requests.
- Walk back to the end of the last long task before that quiet window. That point is Time to Interactive.
This made Time to Interactive sensitive to a single long task or a stray network request. One outlier could push the metric out significantly, which made it noisy and hard to act on.
Why was Time to Interactive deprecated?#
Lighthouse removed Time to Interactive from its report in version 10. The Lighthouse team found that Largest Contentful Paint and Total Blocking Time were more stable, more useful signals, and that Time to Interactive added noise without adding much insight. It no longer contributes to the Lighthouse performance score, though it can still appear in the raw Lighthouse result JSON.
When Time to Interactive was removed, its share of the performance score was reallocated to Cumulative Layout Shift, which rose from 15% to 25%.
What to use instead#
Total Blocking Time (lab)#
In synthetic testing, Total Blocking Time is the responsiveness signal. It totals the blocking portion of long tasks after First Contentful Paint, and it is now weighted at 30% of the Lighthouse performance score. It is more stable than Time to Interactive because it measures total blocking work rather than a single quiet-window boundary.
Interaction to Next Paint (field)#
For real users, Interaction to Next Paint is the Core Web Vital that measures responsiveness. INP became a Core Web Vital on 12 March 2024, replacing First Input Delay, and it reports the latency of real interactions across the whole page visit.