How Third-Party Scripts Affect INP and Ways to Fix It

Ritisha

August 25, 2025

Introduction: A Hidden Culprit in Your Site’s Sluggishness

Not all performance issues are caused by bloated design or backend bottlenecks. In fact, one of the most common, yet overlooked, contributors to slow site performance lies hidden in the very tools marketers and developers often rely on: third-party scripts.

Imagine a site that loads well initially but feels sluggish when a user tries to interact. They click a button—nothing happens. They open a menu—there’s a delay. This is where the Interaction to Next Paint (INP) metric reveals the real story. As the latest Core Web Vitals metric replacing FID, INP measures how quickly your site responds to user input.

If you’ve been trying to improve Core Web Vitals but still see poor scores despite “optimizing everything,” it’s time to look deeper—specifically at how you integrate third-party scripts.

This blog explores the impact of third-party scripts on INP, how to measure that impact, and most importantly, how to fix Core Web Vitals issues they create without sacrificing your business-critical tools. Whether you’re planning a Core Web Vitals optimization strategy or want to know how to improve interaction to next paint, you’ll find actionable answers here.

Google’s Core Web Vitals and INP: A Modern Look at Site Interactivity

What Are Core Web Vitals?

Core Web Vitals are performance metrics introduced by Google to quantify the quality of user experience on the web. As of 2024, they include:

• Largest Contentful Paint (LCP) – measures loading performance
• Cumulative Layout Shift (CLS) – measures visual stability
• Interaction to Next Paint (INP) – measures responsiveness

INP became a stable Core Web Vital in March 2024, replacing First Input Delay (FID), as it better reflects real-world interaction performance.

What Is INP?

Interaction to Next Paint (INP) measures the time between a user interaction (click, tap, keypress) and the next time the page visually updates in response to that interaction. Lower INP scores mean better responsiveness.

A “good” INP is under 200ms, “needs improvement” is 200–500ms, and anything above 500ms is poor.

Why INP Matters

Responsiveness is often the difference between a seamless experience and a frustrating one. Sites with poor INP lose conversions, reduce engagement, and suffer SEO consequences due to Core Web Vitals’ impact on Google rankings.

Third-Party Scripts: What They Are and Why They Matter

Third-party scripts are any scripts that load from external domains—not part of your core application codebase. These include:

• Analytics (e.g., Google Analytics, Hotjar)
• Ads and tracking pixels
• Chat widgets (e.g., Intercom, Drift)
• Social media embeds
• A/B testing and personalization tools
• Tag management systems (e.g., GTM)

Why Use Third-Party Scripts?

They provide critical functionality—marketing, support, analytics, revenue—but at a cost. Every script competes for processing time on the browser’s main thread, affecting your site’s ability to respond quickly to user interactions.

The Impact of Third-Party Scripts on INP

Third-party scripts are notorious for causing long tasks, blocking rendering, and delaying event handlers—all of which degrade INP.

How They Affect the Main Thread

The browser’s main thread handles user input, rendering, and JavaScript execution. When a third-party script runs a heavy task—like initializing a chat widget—it can delay everything else, including visual updates after a user clicks a button.

Common Offenders

• Chat widgets initializing at page load
• Analytics scripts processing before interaction
• Tag managers injecting multiple blocking scripts
• Ads re-rendering content after interaction

Domino Effect on Core Web Vitals Metrics

• LCP: Delays caused by third-party script loading can block rendering of large content.
• CLS: Ads and social embeds can shift layout as they load unpredictably.
• INP: Long tasks or poor event listener management lead to interaction lag.

How to Measure the Impact of Third-Party Scripts on INP

Knowing what’s slowing your site is the first step in core web vitals optimization.

Tools to Use

• Lighthouse: Shows long tasks, script timings, and diagnostics.
• Chrome DevTools:
  1. Performance tab: Analyze long tasks and interaction timings.
  2. Coverage tab: Identify unused JS/CSS from third-party scripts.
• WebPageTest + Request Map: Visualizes how scripts load and interact.
• Real User Monitoring (RUM): SpeedCurve, Calibre, New Relic can help measure Interaction to Next Paint over time and across devices.

Key Metrics to Look At

• Long Tasks (>50ms)
• Time to First Interaction
• Event Timing and Duration
• Script Initiators and Call Stack Tracing

How to Fix Core Web Vitals Impacted by Third-Party Scripts

Optimizing INP doesn’t mean eliminating all third-party tools—it means using them smartly.

Defer or Lazy-Load Non-Critical Scripts

Only load chatbots or marketing tools after the page is interactive or based on user behavior. Lazy-load scripts with IntersectionObserver or defer loading after a user scrolls or clicks.

Use async and defer Attributes Wisely

• async: Loads scripts in parallel but executes them as soon as possible.
• defer: Defers execution until the DOM is fully parsed.
• Use defer for scripts that don’t need to block rendering.

Replace with Lightweight Alternatives

• Use analytics libraries with smaller footprints.
• Replace legacy tag managers with modern, event-driven alternatives.
• Swap heavy chat tools for lighter ones that load on user intent.

Offload to Web Workers

Move non-critical logic (e.g., heavy computations or personalization) to web workers so they don’t block the main thread.

Audit and Remove Unused Scripts

Use DevTools coverage reports to identify and remove redundant or unused third-party scripts.

Safe Integration: Best Practices for Third-Party Scripts

Optimizing isn’t just about fixing today’s issues but preventing tomorrow’s.

Sandbox Scripts in Iframes

Isolate risky third-party content to prevent it from interfering with the main thread or causing layout shifts.

Self-Host When Possible

Download and serve critical third-party scripts from your own domain to reduce DNS lookups and improve control over caching.

Load on Consent

Use a consent management platform to delay loading of marketing or analytics scripts until the user agrees.

Monitor Script Impact Continuously

Set up alerts for spikes in INP or long tasks using RUM tools. Create a feedback loop between developers and marketing.

Building a Core Web Vitals Strategy

Improving INP and other vitals requires a strategic, sustainable approach.

Define Critical User Interactions

Focus optimization efforts on the most common or important user journeys.

Set a Performance Budget

Limit how much JavaScript or third-party code is allowed on your pages. Budget not just for KBs but also long task durations.

Include Performance in CI/CD

Integrate Lighthouse CI or WebPageTest in your build process to catch regressions early.

Educate Teams

Marketing and product teams should understand the tradeoffs of third-party scripts. Provide alternatives and data to support decisions.

Conclusion

Third-party scripts are essential for business operations, but unmanaged, they silently erode performance—especially your INP scores. The good news? With the right tooling, strategy, and mindset, you can maintain functionality while optimizing for Core Web Vitals.

By taking a systematic approach to script loading, isolating and delaying non-essential third-party tools, and continuously monitoring performance, you can significantly improve Core Web Vitals—especially INP—and ensure a responsive, engaging user experience.

If you’re unsure where to start or need expert help navigating these technical waters, consider hiring professionals who specialize in this area. Hire Core Web Vitals consultants to build a sustainable, performance-first strategy tailored to your website’s goals.