Interaction to Next Paint
The Interaction to Next Paint (INP) metric is the successor of the First Input Delay (FID) metric. INP aims to measure the overall delay from user interaction to the moment the next paint or frame was presented, resulting in a perceivable visual change for the user. This visual feedback is important to ensure visitors perceive a website and on-site interactions as fast.
This metric is available in RUM, synthetic monitoring and soft navigations.
INP became part of Google's Core Web Vitals on March 12, 2024.
Optimal responsiveness is important from a user experience perspective, as based on Nielsen Norman Group research users are able to perceive delays when they take longer than 100 milliseconds.
Make sure to read our master article on how to fix Interaction to Next Paint.
Difference with FID
the First Input Delay metric was falling short as it didn't measure the whole process from interaction to next visual change. INP is doing a better job as it is including Input Delay, Processing Time and Presentation Delay, tracking the exact time from user input until next (visual) paint.
Succeeding the FID metric
On January 31st, Google announced that on, March 12 INP will become part of Core Web Vitals. It is replacing the FID metric, meaning that it will then no longer be a Core Web Vitals metric.
Nuances of the Interaction to Next Paint metric
There are some important nuances when it comes to the Interaction to Next Paint metric:
- page life cycle
While the FID is limited to the first interaction, the INP keeps being monitored throughout the whole page life cycle. - Single Page Applications
If your site is a Single Page Application, there are exceptions when it comes to tracking Core Web Vitals across all page navigations. - 98th percentile
This is confusing in combination with the 75th percentile and thus removed by Google from their public resources. However, some sites -such as SPA's and gaming sites- might get way more interactions within a single page life cycle. This could mean they have a higher chance of being penalized. To smoothen this out, Google will remove the highest INP of each 50 interactions, and then report the second highest INP value.
Lighthouse INP versus Core Web Vitals INP
There's a low correlation between the INP score that you would see in Lighthouse or synthetic monitoring in general (both lab data), and Core Web Vitals or Real User Monitoring (both field data). As a matter of fact, the INP metric doesn't exist in lab data. The closest metric is Total Blocking Time.
As Lighthouse won't scroll and won't click on elements (unless configured otherwise), you won't be able to get data from user interaction in lab data tests. That's why INP is not available in Lighthouse.
Continue reading about Interaction to Next Paint
Want to learn more about Interaction to Next Paint? Check our blog, check the web.dev INP article or keep an eye out for any changes to the INP metric.