INP: Optimize Interaction to Next Paint for Better Web Experiences

Understanding INP and Its Importance

Definition of Interaction to Next Paint (INP)

Interaction to Next Paint (INP) is a Core Web Vital metric that measures the responsiveness of a webpage to user interactions. INP officially replaced First Input Delay (FID) as a Core Web Vital on March 12, 2024, marking a significant shift in how Google evaluates user experience ^[1].

The metric captures the full interaction latency by measuring input delay, processing time, and presentation delay combined ^[2]. Unlike simpler metrics, INP evaluates all interactions during a page visit, not just the first one.

This comprehensive approach is crucial because 90% of user time on a page is spent after it loads, making ongoing responsiveness essential for user satisfaction ^[3]. A good INP score is under 200 milliseconds, while scores over 500 milliseconds are considered poor ^[4].

How INP Differs from Other Web Vitals

INP stands apart from other Core Web Vitals by focusing specifically on interactivity throughout the entire page lifecycle. While Largest Contentful Paint (LCP) measures loading performance and Cumulative Layout Shift (CLS) tracks visual stability, INP ensures that user actions receive timely feedback.

The metric uses the 75th percentile of all interactions to determine the overall score, providing a more realistic assessment of user experience ^[5]. The impact of this change has been substantial across the web.

Nearly 600,000 websites failed Core Web Vitals assessments after INP replaced FID, highlighting how many sites struggle with ongoing responsiveness ^[6]. Currently, only 53% of origins achieve good scores for all three Core Web Vitals (LCP, CLS, and INP), with 47% of sites failing the INP metric specifically ^[7][8].

The Impact of INP on User Experience

Poor INP scores directly correlate with frustrated users and lost business opportunities. When interactions feel sluggish or unresponsive, users often abandon tasks or leave sites entirely.

The mobile web has shown significant improvement in this area, with sites achieving good INP scores rising from 55% in 2022 to 74% in 2024 ^[9]. The business impact of INP optimization is measurable and significant.

Page abandonment reduces by 24% when sites meet Core Web Vitals thresholds, directly affecting conversion rates and user engagement ^[10]. Every 100 milliseconds of delay can reduce conversions by 7%, making INP optimization critical for business success ^[11].

Measuring and Analyzing INP

Tools for Tracking INP Metrics

Measuring INP requires both lab and field tools to get a complete picture of performance. Field data from real users provides the most accurate representation of actual INP performance, while lab tools help identify and debug specific issues.

Chrome User Experience Report (CrUX) aggregates field data over a 28-day period, providing a stable baseline for performance tracking ^[12]. Total Blocking Time (TBT) serves as the recommended lab proxy for INP, helping developers identify potential responsiveness issues during development ^[13].

Tools like PageSpeed Insights, Chrome DevTools, and Web Vitals extension provide both real-time and historical INP data. Firefox recently added INP support in version 144 (October 2025) as part of the Interop 2025 initiative, expanding measurement capabilities across browsers ^[14].

Interpreting INP Data

Understanding INP thresholds is essential for proper optimization. The metric classifies performance into three categories: good (less than 200ms), needs improvement (200ms-500ms), and poor (above 500ms) ^[15][16][17].

These thresholds apply across all device types, though actual performance varies significantly based on device capabilities and network conditions. Geographic and device differences significantly impact INP scores.

In the United States, typical mobile INP at the 75th percentile is 100ms, while desktop achieves around 50ms ^[18][19]. However, in countries like Nigeria, typical mobile INP jumps to 275ms, demonstrating how regional infrastructure affects performance ^[20].

Setting Realistic INP Benchmarks

Establishing appropriate INP targets requires understanding your audience's device capabilities and network conditions. While the 200ms threshold represents a good universal target, competitive analysis within your industry provides more relevant benchmarks. E-commerce sites, for instance, often aim for sub-100ms INP to maximize conversions.

The web performance market is experiencing rapid growth, expanding from $5. 97 billion in 2025 to a projected $9. 03 billion by 2030, representing 51% growth ^[21].

This investment reflects the critical importance of metrics like INP for business success. Organizations should set incremental improvement goals, as even small gains can yield significant user experience benefits.

Common Causes of Poor INP Performance

JavaScript Execution Bottlenecks

JavaScript's run-to-completion model creates one of the most significant challenges for INP optimization ^[22]. When JavaScript executes, it blocks the main thread, preventing the browser from responding to user interactions.

Any task taking 50ms or more is considered a long task that degrades INP performance ^[23]. Heavy computational work, inefficient algorithms, and excessive third-party scripts compound these bottlenecks.

Modern web applications often load multiple analytics, advertising, and functionality scripts that compete for main thread time. Each script adds potential delay to user interactions, creating cumulative performance degradation.

Inefficient DOM Manipulation

DOM operations trigger expensive browser calculations that impact INP scores. Reflows, which recalculate element positions and dimensions, are particularly expensive compared to simple repaints ^[24].

Frequent DOM updates, especially within loops or event handlers, can create significant interaction delays. Batch DOM operations and virtual DOM implementations help mitigate these issues.

However, many developers still write code that triggers multiple reflows per interaction. Common culprits include reading computed styles immediately after writing them, modifying multiple style properties separately, and using JavaScript for animations instead of CSS.

Resource-Intensive Animations and Transitions

Animation performance significantly affects INP, particularly on lower-end devices. CSS animations use 14. 11% more memory than transitions and achieve 16.

80% lower frame rates under stress, dropping to just 12. 04 FPS ^[25][26]. These performance hits directly impact the browser's ability to respond to user interactions promptly.

Not all CSS properties perform equally for animations. Properties like transform and opacity are GPU-accelerated and don't trigger reflows, making them ideal for smooth animations ^[27]. In contrast, animating properties like width, height, or position forces expensive layout recalculations that degrade INP scores.

Strategies to Optimize INP

Minimizing Main Thread Blocking

Breaking up long tasks is the most effective strategy for improving INP. Tasks should be kept under 50ms to maintain responsive interactions, with potential INP improvements of approximately 200ms when properly implemented ^[28]. The scheduler.

postTask() API provides three priority levels—user-blocking, user-visible, and background—allowing developers to optimize task execution order ^[29]. Code splitting reduces initial JavaScript bundle sizes by 40-70% on single-page applications, significantly reducing main thread blocking during initial load ^[30]. This technique ensures users download only the code needed for immediate interactions, with additional functionality loaded on demand.

Combined with lazy loading, which can reduce initial page weight by 30-50% on image-heavy sites, these strategies create more responsive experiences ^[31].

Implementing Efficient Event Handlers

Event handler optimization directly impacts INP scores by reducing processing time for user interactions. Debouncing and throttling techniques prevent excessive function calls, particularly for high-frequency events like scrolling or resizing.

These patterns ensure the browser maintains responsiveness even during rapid user input. Real-world implementations demonstrate the effectiveness of event handler optimization.

RedBus improved INP by 72% through systematic optimization, resulting in a 7% increase in sales ^[32]. Similarly, Fotocasa saw 27% growth in key metrics following INP improvements, while Trendyol achieved a 50% INP reduction and 1% CTR increase ^[33][34].

Using Browser APIs for Smoother Interactions

Modern browser APIs provide powerful tools for INP optimization. The Long Animation Frames API (LoAF), shipped in Chrome 123, helps developers identify and debug interaction bottlenecks ^[35].

This API provides detailed timing information about long-running animations and their impact on responsiveness. Web Workers and off-main-thread processing dramatically improve INP by moving heavy computations away from the UI thread.

Partytown, a library that relocates third-party scripts to web workers, has demonstrated Lighthouse score improvements from approximately 70 to 99 ^[36]. Service Workers can also pre-cache critical resources and handle network requests without blocking user interactions.

Advanced INP Considerations

Integrating INP Optimization into Development Workflow

INP optimization must be embedded throughout the development lifecycle, not treated as an afterthought. With over 70% of new applications using low-code/no-code tools by 2025, even non-traditional developers need INP awareness ^[37].

Automated testing should include INP checks alongside other performance metrics to catch regressions early. Performance budgets specifically for INP help teams maintain standards across releases.

Setting maximum thresholds for interaction latency and automatically flagging violations prevents gradual performance degradation. Continuous monitoring through real user monitoring (RUM) tools provides ongoing validation of INP improvements in production environments.

Balancing INP with Other Performance Metrics

Optimizing INP sometimes conflicts with other performance goals, requiring careful balance. Currently, only 47% of sites meet all Core Web Vitals thresholds, with 8-35% conversion losses attributed to poor Core Web Vitals performance ^[38][39].

Sites that successfully move from poor to good Core Web Vitals see conversion rate increases of 25%, demonstrating the importance of holistic optimization ^[40]. Hotstar's case study exemplifies successful balance, achieving a 61% INP reduction that resulted in a 100% increase in weekly card views ^[41].

This dramatic improvement came from coordinating INP optimization with overall performance strategy rather than focusing on a single metric in isolation.

Future-Proofing Your Web Application for INP

The web performance landscape continues evolving rapidly, with estimated time savings of 30,000 years from web performance improvements in 2024 alone ^[42]. Future-proofing requires building flexibility into architectures to accommodate new optimization techniques and browser capabilities as they emerge.

Progressive enhancement strategies ensure applications remain responsive across diverse devices and network conditions. Starting with a performant baseline and layering complexity based on device capabilities helps maintain good INP scores across all user segments.

Regular performance audits and staying current with browser API developments ensure long-term INP optimization success.