Time to Interactive

The Time to Interactive (TTI) metric aims to identify cases where a page looks interactive but actually isn't. A fast TTI helps ensure that the page is usable.

This metric is only available in synthetic monitoring.
TTI was removed from Lighthouse UI in v10.

The TTI used to be visible in Lighthouse, but got removed in Lighthouse v10. It also doesn't contribute to the performance score anymore. The TTI data will still be there in the JSON result of a Lighthouse test.

Nuances of the Time to Interactive metric

There are some interesting nuances when it comes to the Time to Interactive metric:

  • TTI in short
    The TTI metric measures the time from when the page starts loading to when its main sub-resources have loaded
  • idle thread
    Additionally, it also waits for the last Long Task to be finished and then waits for at least 5 seconds of network and main thread inactivity
  • performance score hacking
    This way of measuring led to site owners cheating the score by delaying their Google Tag Manager or all JavaScript by 5 seconds.
    This way, it would not become part of what Lighthouse was seeing, and one would get a better score.

With TTI being removed, most Lighthouse scores should have improved as TTI metric contributed to the Lighthouse score for 10%. So, by loading many JS with different triggers, one could get a very high TTI score. This then ended up in massively impacting the overall Lighthouse score as well.

Lighthouse TTI versus Core Web Vitals TTI

We can be short here. There is no TTI metric in Core Web Vitals.

The best Core Web Vitals equivalent is the Interaction to Next Paint (INP) metric, as that is tracking the time from user interaction to the next moment of a possible paint. As this involves long tasks too, one should nowadays be looking into the INP metric.

Continue reading about Time to Interactive

Want to learn more about Time to Interactive? Check the web.dev TTI article.