What's it all about?

The facts upfront: In March 2024, Google is replacing First Input Delay (FID) with Interaction to Next Paint (INP).

First Input Delay (FID) and its limitations

First Input Delay (FID) is one of the core metrics within the Core Web Vitals developed by Google to evaluate user experience on websites. FID measures the delay between the moment a user first interacts with a website, for example by clicking on a link or filling out a form, and the moment the website actually responds to this interaction.

FID was introduced to capture a crucial element of usability: the responsiveness of a website to user actions. A low FID means that a website responds quickly and smoothly to user interactions, resulting in a positive user experience.

The limitations of FID: Why INP is needed

Although FID undoubtedly provides valuable insights into website performance, there are some significant limitations:

  • Limited consideration of all interactions: FID only measures the delay until the first interaction. While this is important, as a delayed response to the first action can increase user frustration, it does not take into account all interactions during a website visit. Users often perform multiple actions on a page, and it is important to capture the latency of all interactions.

  • Missing information about overall performance: FID focuses exclusively on the delay until interaction and neglects other important factors such as time to content display or website stability after interaction. While a fast response is important, it alone does not provide a complete picture of user experience.

  • Capturing only the first interaction: FID ignores all subsequent interactions. If a website responds slowly after the first action, this can significantly impact user experience, but is not captured by FID.

  • Limited significance for pages with many interactions: On websites with numerous interactions, FID can be insufficient as it only measures the delay until the first interaction. In such cases, there can be significant delays in subsequent actions that increase user frustration but remain unconsidered by FID.

Interaction to Next Paint (INP)

Interaction to Next Paint (INP) represents a new metric that captures the latency of all user interactions during a website visit. It considers not only the duration until the page is loaded, but especially the time after interaction. Because: Approximately 90 percent of the time users spend on a website is after the page has loaded.

INP determines the highest value of all interactions, especially on pages with many interactions. This value reflects the page's response time to user interactions.

INP compared to FID

INP differs from First Input Delay (FID), another Core Web Vitals metric. While FID only measures the first interaction, INP captures all interactions during the page visit. Additionally, FID only considers the input delay and not the processing time of event handlers or the duration until the next frame is displayed.

Good and bad INP values

Google has established guidelines for INP values:

  • An INP under 200 milliseconds is considered good responsiveness.
  • An INP between 201 and 500 milliseconds means that improvements are necessary.
  • An INP of over 500 milliseconds indicates poor responsiveness.

It is important to note that Google does not use the highest value to calculate INP, as websites can have varying numbers of interactions. Instead, Google looks at the high percentile to ensure that most interactions are acceptably fast.

Why is INP so important?

INP not only influences website performance but also has direct effects on user experience and conversion rates. Studies have shown that users are more willing to stay on a website and perform actions such as completing a purchase or filling out a form when the page responds quickly and smoothly to their inputs.

Furthermore, a good INP value can provide a competitive advantage: Websites that respond faster to user interactions not only offer a better experience but also stand out positively from the competition.

Measuring and optimizing INP

You can obtain INP both as lab data and as field data. Lab data is provided by various tools such as the Lighthouse npm module, Lighthouse User Flows, or the Web Vitals extension for Google Chrome. Field data can be retrieved via Google PageSpeed Insights or the Chrome User Experience Report (CrUX).

To optimize INP, you should take measures during page loading, such as removing unnecessary JavaScript code or code splitting. After the page has loaded, you can use the postTask API to prioritize tasks, or identify and remove slow third-party JavaScript code.

Additional optimization measures:

  • Remove unnecessary JavaScript code: Reduce the size and complexity of your JavaScript files to shorten loading time.
  • Eliminate third-party code: Review which third-party scripts are active during loading and remove non-essential scripts.
  • Optimize event handler processing time: Ensure that your event handlers work efficiently and quickly to minimize delays.

Challenges with CMS systems

Particularly traditional CMS systems like WordPress could be affected by INP when there is no custom development and unnecessary scripts are loaded. A revision of the website structure and a reduction of unnecessary code can provide a remedy here. Poorly developed plugins or bloated WordPress themes can also negatively impact responsiveness.

What is not included in the evaluation of INP?

Interactions such as scrolling and hovering have no influence on the INP value. Instead, only those interactions where visual feedback is expected are considered, such as keyboard clicks or page changes.

Conclusion

Interaction to Next Paint (INP) is an important metric for the page experience of websites. Website operators should familiarize themselves with this metric and take measures to improve the responsiveness of their pages. A better user experience and potentially higher rankings are the reward for efforts to optimize INP.