Interaction to Next Paint (INP) in Core Web Vitals

Interaction to Next Paint (INP) is a Core Web Vital metric introduced to measure the responsiveness of web pages. It provides insights into how quickly a page reacts to user interactions (such as clicks, taps, and keypresses) and reports a single latency value that represents the page’s overall responsiveness. INP is particularly valuable because it observes interactions throughout the entire user session, unlike its predecessor, First Input Delay (FID), which only measured the responsiveness of the first user interaction.

Why INP Matters ?

• Post-load Interactivity: Chrome usage data reveals that users spend 90% of their time on a page after it loads, making the measurement of interactivity throughout the lifecycle critical.
• Responsiveness Metric: INP tracks delays in visual feedback to user interactions, helping developers identify and optimize areas where users might experience sluggish responses.
• User Experience: Delays in responsiveness can make users think a page is broken, leading to frustration and potential loss of engagement.

How INP Works:

1. Definition of Interaction:
   • INP measures latency for clicks, taps, and keypresses. It tracks the entire lifecycle of each interaction, including:
     • Input delay: Time between user input and the browser starting to process it.
     • Event handling: Execution time of event handlers.
     • Presentation delay: Time until the browser presents the next frame with visual feedback.
2. Final INP Value:
   • The final INP value is determined by observing the longest latency among all interactions during the session. Outliers are ignored to ensure the result reflects a typical user experience.
3. Difference from FID:
   • FID only measured the input delay of the first interaction during page load.
   • INP covers all interactions throughout the page’s lifecycle, providing a more comprehensive measure of responsiveness.

What is a Good INP Score?

• Good: ≤200ms
• Needs Improvement: 201ms–500ms
• Poor: >500ms

Achieving a low INP score indicates that your page consistently provides fast responses to user interactions.

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How to Measure INP:

In the Field:

• Field data is the most reliable method for assessing INP, as it reflects real-world user experiences.
• Tools:
  • Chrome User Experience Report (CrUX): Provides INP data at both origin and URL levels.
  • Real User Monitoring (RUM): Offers detailed interaction data, helping to pinpoint causes of slow responsiveness.

In the Lab:

• Lab tools can simulate interactions to measure INP, but results depend on the specific user flows tested.
• If field data is unavailable, Total Blocking Time (TBT) can serve as a proxy for INP during testing.

Improving INP:

1. Identify Slow Interactions:
   • Use field data to locate problematic interactions.
   • Supplement with lab testing to reproduce and analyze slow interactions.
2. Optimize Key Areas:
   • Reduce JavaScript execution time to free up the main thread.
   • Minimize the impact of long tasks by breaking them into smaller chunks.
   • Implement efficient rendering techniques to ensure quick updates after interactions.

FAQs about INP:

Q1: What happens if no INP value is reported?

• This occurs when:
  • The user doesn’t interact with the page using measurable gestures (e.g., only scrolling or hovering).
  • The page is accessed by bots or headless browsers.

Q2: How is INP different from FID?

• FID measures only the first interaction’s input delay, focusing on page load. INP encompasses all interactions, offering a broader view of the page’s responsiveness throughout its lifecycle.

Q3: Can INP be measured for iframe interactions?

• Yes, INP measures interactions within iframes as part of the top-level page’s responsiveness, ensuring a complete view of the user experience.

Q4: What tools can be used for INP measurement?

• Field Data: CrUX, RUM solutions.
• Lab Testing: Tools like Lighthouse or WebPageTest.

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Key Takeaways:

• INP reflects overall page responsiveness, measuring latency from interaction to visual feedback.
• A low INP (≤200ms) ensures users experience minimal delays in interaction.
• Field data provides the most accurate insights for optimization, while lab testing helps replicate and resolve issues.
• Optimizing INP involves reducing long tasks, improving event handling, and ensuring the browser can present frames promptly after interactions.

By focusing on improving INP, developers can significantly enhance user satisfaction and engagement.