Core Web Vitals Optimization 2025: Boost INP, LCP & CLS

Introduction: The Evolution of Page Experience Metrics

In the modern web ecosystem, user experience is inseparable from page performance. Google’s Core Web Vitals—a set of standardized metrics to quantify real-world user experience—have evolved significantly over the years. Initially, the focus was on Largest Contentful Paint (LCP), Cumulative Layout Shift (CLS), and First Input Delay (FID).

However, as websites became increasingly JavaScript-heavy and user interactions more complex, FID alone failed to capture the full responsiveness picture. Enter Interaction to Next Paint (INP)—a next-generation responsiveness metric that now replaces FID as a Core Web Vital starting in 2025.

This comprehensive guide explains what INP is, why it matters for both developers and SEO professionals, how to measure and optimize it using real-world tools, and how it compares with older metrics like FID and TBT. By the end, you’ll understand how to achieve an excellent INP score to deliver smoother, faster, and more engaging user experiences—critical for both SEO rankings and conversion rates.

1. Understanding Interaction to Next Paint (INP): What It Is and Why It Matters

What Is Interaction to Next Paint (INP)?

Interaction to Next Paint (INP) measures the responsiveness of your web page to user interactions. It quantifies how long it takes from the moment a user interacts (click, tap, or key press) until the next frame is visually rendered, updating the interface in response to that input.

In simpler terms, INP evaluates how quickly your page reacts after the user does something—reflecting the real-world experience of responsiveness.

• Good INP: ≤ 200 milliseconds
• Needs Improvement: 200–500 milliseconds
• Poor INP: > 500 milliseconds

Unlike earlier metrics, INP observes all interactions during a user’s page visit, not just the first one. This makes it more representative of a site’s overall responsiveness, especially on JavaScript-rich sites.

INP vs FID — The Core Difference

FID measured only input delay—the time between a user’s first interaction and the browser’s response start. It didn’t account for visual feedback or later interactions.

INP, on the other hand, measures end-to-end interaction latency, including the event handler, processing, and painting stages. This makes INP a more accurate indicator of how responsive your page feels to users throughout their entire session.

Why Google Replaced FID with INP

Google’s decision to replace FID with INP as a Core Web Vital was based on data showing that FID often underestimated poor responsiveness.

Key reasons for the shift:

1. Comprehensive coverage: INP reflects the slowest interactions during a session.
2. Real-world accuracy: It aligns better with field data from Chrome User Experience Report (CrUX).
3. User-centric: INP directly represents perceived performance, not just technical delay.

As Google’s Search Central team confirmed, “INP offers a more complete view of responsiveness across the user’s entire experience.”

How INP Impacts SEO and User Experience

A poor INP score signals that users experience laggy, unresponsive interfaces—leading to frustration, higher bounce rates, and lower engagement.

From an SEO standpoint, INP contributes to the Page Experience ranking signal. Pages with excellent INP not only deliver smoother interactions but also earn better engagement metrics (e.g., dwell time, reduced pogo-sticking), which can indirectly improve search rankings.

2. How to Measure INP: Best Tools and Metrics

Using Google Lighthouse to Evaluate INP

Google Lighthouse, built into Chrome DevTools, provides a quick lab analysis of INP performance.

Steps:

1. Open your page in Chrome.
2. Press Ctrl+Shift+I (or Cmd+Opt+I on Mac) to open DevTools.
3. Navigate to the Lighthouse tab and run an audit.
4. Under Performance, find Interaction to Next Paint (INP).

Lighthouse simulates interactions and reports your INP score with improvement recommendations like reducing long tasks or deferring scripts.

Chrome DevTools for Real-Time INP Analysis

Chrome DevTools’ Performance Panel offers deeper insight:

1. Record page load and interactions.
2. View Main Thread tasks that contribute to high INP.
3. Identify long-running JavaScript or layout shifts causing delays.

You can visually correlate input events to frame renders to see exactly where responsiveness breaks down.

PageSpeed Insights and Real-User Data (RUM)

Unlike Lighthouse (lab data), PageSpeed Insights (PSI) pulls field data from Chrome UX Report (CrUX)—real metrics from actual users.

Field Data Thresholds:

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

This dual reporting—lab + field—helps validate improvements across real devices and networks.

Web-Vitals JS Library & CrUX Data

For ongoing monitoring, integrate the Web-Vitals JavaScript library:

import {onINP} from 'web-vitals';

onINP(console.log);

It logs INP in real time, letting you monitor user interactions and performance bottlenecks.

You can also pull INP data via the CrUX API, aggregating real-world performance for continuous improvement dashboards.

3. Top 10 Ways to Improve INP for a Smoother User Experience

Optimizing INP requires minimizing main-thread blocking and ensuring fast feedback after any user action. Below are the top 10 technical methods used by web performance experts.

1. Reduce Main-Thread Blocking Time

Keep your main thread clear by minimizing heavy JavaScript execution.

• Split long tasks into smaller chunks using requestIdleCallback() or async functions.
• Use Web Workers for CPU-intensive computations.

2. Optimize and Split Long Tasks (>50ms)

Long tasks block the browser from responding. Use Chrome DevTools’ “Performance” tab to find them, then break large scripts into smaller modules.

3. Use Async & Defer for Non-Critical JavaScript

Scripts loading synchronously block rendering.
Example:

<script src="analytics.js" defer></script>

This ensures critical rendering isn’t blocked by non-essential scripts.

4. Lazy Load Images & Videos

Implement lazy loading to defer off-screen media until it’s needed:

<img src="image.jpg" loading="lazy" alt="Example">

This reduces render-blocking requests, improving first and subsequent interactions.

5. Preload Critical Assets

Use <link rel="preload"> for above-the-fold fonts, CSS, and images to improve interaction readiness.

6. Reduce JavaScript Payloads

Minify, compress, and tree-shake your JS bundles. Tools like Webpack, esbuild, or Rollup can remove unused code, shrinking payloads and cutting parsing time.

7. Use Web Workers for Heavy Computation

Move expensive scripts off the main thread:

const worker = new Worker('worker.js');
worker.postMessage('start');

This allows UI interactions to remain responsive during data processing.

8. Optimize Event Handlers and Listeners

Avoid attaching multiple listeners to frequently triggered events (like scroll or mousemove). Use debouncing or throttling to limit execution frequency.

9. Improve Rendering Pipeline (CSS & Paint)

Simplify complex CSS selectors, reduce reflows, and minimize layout thrashing. Tools like Performance Insights in Chrome can reveal layout bottlenecks.

10. Monitor INP Continuously Using RUM Tools

Integrate tools like SpeedCurve, Datadog RUM, or Google Analytics 4 custom metrics to continuously track INP and alert when thresholds are exceeded.

4. Real-World INP Optimization Case Study

Let’s explore how INP optimization works in practice using a hypothetical e-commerce website.

Initial Audit and Metrics

Before optimization:

Using Lighthouse and PSI, the main issue identified was long JavaScript tasks blocking input processing and visual updates.

Optimization Steps Implemented

1. Code-splitting large JS bundles into smaller chunks.
2. Deferring non-critical third-party scripts (analytics, ads).
3. Using async image loading and font preloading.
4. Offloading product filter computations to a Web Worker.
5. Debouncing input events in the search filter.

Post-Optimization Results

After optimization:

That’s a 68% improvement in INP responsiveness, directly improving user experience and engagement metrics.

SEO & Engagement Improvements

After optimizing INP:

• Bounce rate dropped by 22%.
• Average session duration increased by 18%.
• Conversion rate improved by 9%.
• Core Web Vitals report in Google Search Console turned all green.

Better INP translated to smoother interactivity, happier users, and improved organic rankings over time.

5. INP vs. FID vs. TBT: Which Metric Matters Most in 2025?

Metric Evolution Timeline: FID → TBT → INP

• FID (First Input Delay) — introduced in 2020 to measure input delay.
• TBT (Total Blocking Time) — used as a lab proxy for FID, capturing blocking during load.
• INP (Interaction to Next Paint) — adopted in 2024–2025 as the definitive field responsiveness metric.

Comparative Table: FID vs TBT vs INP

Why INP Is the Most Reliable Interaction Metric in 2025

INP combines the strengths of both FID and TBT while overcoming their limitations. It focuses on real-world interactivity—covering not just page load but entire user sessions.

With the global web shifting toward single-page applications (SPAs) and JavaScript-heavy interfaces, INP ensures that even post-load interactions are measured and optimized.

Key Takeaways for Developers & SEO Professionals

1. Prioritize INP as your main performance KPI for responsiveness.
2. Use TBT in lab tests to anticipate INP improvements.
3. Keep your Core Web Vitals dashboard updated with CrUX and Search Console data.
4. Remember: Fast responsiveness → higher engagement → better SEO performance.

6. Final Thoughts: The Road Ahead for Page Experience Optimization

As the web becomes more interactive and user expectations continue to rise, Interaction to Next Paint (INP) has emerged as the most important signal of how responsive your site truly feels.

Developers should embrace tools like Chrome DevTools, PageSpeed Insights, and Web-Vitals JS to continuously measure INP in production. Regular audits, code hygiene, and front-end optimization best practices will ensure that your site remains competitive across both performance and search visibility.

Optimizing INP is not just about pleasing Google’s algorithms—it’s about delivering faster, smoother experiences that keep users engaged, satisfied, and converting.

Frequently Asked Questions (FAQ)

Q1: What is a good INP score?
A good INP score is ≤200 milliseconds. Anything above 500ms is considered poor.

Q2: How is INP different from FID and TBT?
FID measures the first interaction delay, TBT measures blocking during load, while INP measures all interaction delays across the page lifecycle.

Q3: How can I test INP on my website?
Use PageSpeed Insights, Lighthouse, or the Web-Vitals JS library to test and monitor INP both in lab and real-user environments.

Q4: Does INP affect Google ranking?
Yes—INP is now a Core Web Vital, part of Google’s Page Experience signal, influencing SEO indirectly through user experience quality.