By Shad Super in Questions —

What Is Interaction to Next Paint (INP), and How Does It Differ From First Input Delay (FID) in Measuring User Interaction Responsiveness?

Summary

Interaction to Next Paint (INP) is a newer web performance metric that evaluates a page's overall responsiveness to user interactions by considering the latency of all interactions throughout the page load lifecycle. In contrast, First Input Delay (FID) measures only the delay between a user's first interaction and the browser's response. Understanding the differences between these metrics can help improve user experience by addressing interaction responsiveness more comprehensively.

Understanding Interaction to Next Paint (INP)

Definition and Purpose

Interaction to Next Paint (INP) is a metric introduced by Google to assess a web page's responsiveness to user interactions. Unlike First Input Delay (FID), which measures only the first interaction, INP looks at all interactions that occur during the page's lifecycle. The goal is to provide a more holistic view of interaction performance, ensuring that users experience smooth and responsive interactions throughout their session.

How INP Works

INP measures the time it takes from when a user interacts with the page (e.g., clicking a button, typing in a text box) to when the next frame is painted on the screen. It tracks the duration of these interactions and identifies the longest one as the INP value. This comprehensive approach allows developers to identify and address performance issues more effectively.

First Input Delay (FID)

Definition and Purpose

First Input Delay (FID) measures the time from when a user first interacts with a web page—such as clicking a link, tapping a button, or using a JavaScript-driven control—to the time when the browser responds to that interaction. FID is part of Google's Core Web Vitals and focuses on measuring responsiveness during the initial load phase.

Limitations of FID

The primary limitation of FID is its narrow focus—it only measures the delay of the very first interaction. This means it may not capture performance issues that occur later during a user's session, potentially missing other important latency problems that could affect the overall user experience.

Key Differences Between INP and FID

Scope of Measurement

INP evaluates all user interactions, providing a more comprehensive assessment of interaction latency throughout the entire session. FID, on the other hand, measures only the first user interaction, offering a snapshot of initial responsiveness.

Assessment Criteria

INP calculates the duration of each user interaction and selects the longest duration as the INP value. This helps identify worst-case scenarios in user interaction responsiveness. FID measures the delay from the first interaction to when the browser begins processing event handlers, giving an initial but limited view of responsiveness.

Practical Implications

Improving INP can involve optimizing performance across the entire user session, including reducing JavaScript execution times, optimizing rendering paths, and improving server response times. Enhancing FID typically focuses on optimizing the critical rendering path, reducing initial JavaScript execution, and improving server response times at the initial load.

Examples and Visualization

Example of INP

Consider a user navigating a single-page application (SPA). Assume the user interacts multiple times—scrolling, clicking buttons, and opening menus. INP tracks each of these interactions' durations from interaction to the next visual change (paint) and identifies the interaction with the longest delay to reflect INP.

Example of FID

Imagine the same SPA scenario. The FID metric would only measure the delay experienced during the first click or user action once the page is partially loaded. It does not account for subsequent interactions, possibly overlooking delays that occur later.

Conclusion

Both INP and FID are important metrics for evaluating web performance, but they serve different purposes. INP provides a more comprehensive view of a page's responsiveness by considering all interactions, whereas FID focuses on the first interaction's delay. By understanding and optimizing for both metrics, developers can significantly enhance the overall user experience.

References