Advanced LCP and INP Optimization: CDN Strategies and Real User Monitoring

In the first part of this series, we covered the fundamentals of Core Web Vitals optimization. Part 2 focused on image optimization strategies. Now, let’s dive into advanced techniques that professional developers use to squeeze every millisecond out of their page performance: resource prioritization, task scheduling, CDN strategies, and real user monitoring.

These techniques go beyond the basics and require understanding how browsers prioritize resources, how JavaScript executes on the main thread, and how to leverage global infrastructure and real-world data to continuously improve performance.

important
This article assumes you’ve already implemented the fundamental optimizations from Parts 1 and 2. If you haven’t optimized images, reduced transfer sizes, or addressed obvious bottlenecks, start there first—these advanced techniques build upon that foundation.

Advanced LCP Optimization with Resource Hints

Understanding the Fetch Priority API

The Fetch Priority API (fetchpriority attribute) gives you fine-grained control over resource loading priority. While browsers are generally good at prioritizing resources, they can’t always know which resources are most critical to your specific use case.

The fetchpriority attribute accepts three values:

high: Boost priority relative to other resources of the same type
low: Reduce priority relative to other resources of the same type
auto (default): Let the browser decide

note
fetchpriority is a hint, not a directive. The browser will try to respect your preference but may override it based on other factors like network conditions or resource contention.

Boosting LCP Image Priority

If you’ve identified your LCP element (use Chrome DevTools or PageSpeed Insights), you can dramatically improve load time by setting it to high priority:

<!-- LCP image with high priority -->
<img 
  src="/hero-image.webp" 
  alt="Product showcase" 
  fetchpriority="high"
  width="1200"
  height="600"
>

This tells the browser to start downloading the LCP resource immediately, at the same time as critical CSS and JavaScript, rather than waiting until after those resources have been discovered and prioritized.

tip
For LCP background images loaded via CSS, use a <link rel="preload"> with fetchpriority="high":
<link 
  rel="preload" 
  as="image" 
  href="/hero-bg.webp" 
  fetchpriority="high"
>

Deprioritizing Non-Critical Images

Just as important as boosting critical resources is reducing priority for non-critical ones. For example, images in a carousel that aren’t initially visible:

<ul class="carousel">
  <!-- First slide: high priority (LCP candidate) -->
  <img src="/slide-1.jpg" fetchpriority="high" alt="Featured product">
  
  <!-- Hidden slides: low priority -->
  <img src="/slide-2.jpg" fetchpriority="low" alt="Product 2">
  <img src="/slide-3.jpg" fetchpriority="low" alt="Product 3">
  <img src="/slide-4.jpg" fetchpriority="low" alt="Product 4">
</ul>

important
Don’t use loading="lazy" on your LCP image—it will delay loading until the browser confirms the image is in the viewport, adding unnecessary input delay. Use fetchpriority="high" instead.

Preconnect to Critical Origins

When your LCP resource is hosted on a different origin (like a CDN or image service), the browser needs to establish a connection before it can download the resource. This involves DNS lookup, TCP handshake, and TLS negotiation—easily 100-300ms on mobile networks.

Use <link rel="preconnect"> to start these connections early:

<head>
  <!-- Preconnect to CDN hosting LCP image -->
  <link rel="preconnect" href="https://cdn.example.com">
  
  <!-- Preconnect to font provider -->
  <link rel="preconnect" href="https://fonts.googleapis.com">
  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
</head>

tip
Only preconnect to 2-3 critical origins. Each preconnect consumes resources, and too many can actually slow down your page by competing for bandwidth during the critical initial load phase.

bonus
According to Google’s research, using fetchpriority="high" on the LCP image can improve LCP by 0.5-1.2 seconds on slow connections, as the image starts downloading immediately rather than waiting in the queue behind less critical resources.

Advanced INP Optimization with Task Scheduling

Interaction to Next Paint (INP) measures how quickly your page responds to user interactions. Poor INP almost always stems from long tasks blocking the main thread. Let’s explore advanced techniques to break up these tasks and keep your UI responsive.

Understanding Long Tasks

Any JavaScript execution that runs for more than 50 milliseconds is considered a long task. When a long task is running, the browser cannot respond to user input—clicks, taps, and keystrokes are queued up waiting for the main thread to become available.

// ❌ Bad: Blocks the main thread for a long time
function processAllData(items) {
  for (const item of items) {
    // Complex processing for each item
    calculateMetrics(item);
    updateState(item);
    validateData(item);
  }
}

The solution is to break long tasks into smaller chunks, yielding control back to the browser between chunks.

Using scheduler.yield() for Yielding

The modern, recommended approach is scheduler.yield() (available in Chrome 129+, Firefox 142+). Unlike older techniques, it maintains execution priority so your code resumes before other lower-priority tasks:

// ✅ Good: Yields to allow browser to handle interactions
async function processAllData(items) {
  for (const item of items) {
    // Process the item
    calculateMetrics(item);
    updateState(item);
    validateData(item);
    
    // Yield to the main thread
    await scheduler.yield();
  }
}

important
scheduler.yield() is not yet supported in all browsers (notably Safari as of early 2025). Always include a fallback for cross-browser compatibility.

Cross-Browser Yielding Pattern

Here’s a production-ready yielding function that works across all browsers:

function yieldToMain() {
  // Use scheduler.yield() if available (Chrome 129+, Firefox 142+)
  if (globalThis.scheduler?.yield) {
    return scheduler.yield();
  }
  
  // Fallback to setTimeout for other browsers
  return new Promise(resolve => {
    setTimeout(resolve, 0);
  });
}

// Usage
async function processLargeDataset(data) {
  for (const item of data) {
    processItem(item);
    
    // Yield to keep UI responsive
    await yieldToMain();
  }
}

tip
For even better cross-browser support, you can use the scheduler-polyfill package, which provides a complete implementation of the Scheduler API.

Batching Work to Reduce Overhead

Yielding after every single operation can introduce overhead. A smarter approach is to batch work and only yield when you’ve been running for a certain amount of time:

async function processWithDeadline(items, deadline = 50) {
  let lastYield = performance.now();
  
  for (const item of items) {
    // Process the item
    processItem(item);
    
    // Only yield if we've exceeded the deadline
    const now = performance.now();
    if (now - lastYield > deadline) {
      await yieldToMain();
      lastYield = performance.now();
    }
  }
}

This approach processes as many items as possible within the 50ms budget, then yields. It provides a good balance between responsiveness and efficiency.

Yielding Strategically in Event Handlers

For interactive features, you want to prioritize user-visible updates and defer background work:

// ✅ Structured for optimal INP
async function handleFormSubmit(event) {
  event.preventDefault();
  
  // 1. Critical user-facing updates (run immediately)
  showLoadingSpinner();
  disableSubmitButton();
  
  // 2. Yield to allow the loading indicator to render
  await yieldToMain();
  
  // 3. Perform background work in subsequent tasks
  const formData = collectFormData();
  await yieldToMain();
  
  const validated = validateFormData(formData);
  await yieldToMain();
  
  await sendToServer(formData);
  await yieldToMain();
  
  // 4. Update UI with results
  showSuccessMessage();
  enableSubmitButton();
}

note
By yielding immediately after showing the loading spinner, we ensure the browser can paint that visual feedback before the blocking work begins. This makes the interaction feel instant to users.

CDN Strategies for Core Web Vitals

Content Delivery Networks (CDNs) are essential for optimizing Core Web Vitals, particularly LCP. A well-configured CDN can improve TTFB from 500-1000ms down to 50-150ms by:

Reducing latency: Servers geographically closer to users = faster Time to First Byte (TTFB)
Caching: Cached resources load instantly, eliminating network time entirely

Essential CDN Configuration

Set aggressive cache durations for versioned static assets:

// Express.js example
app.use(express.static('public', {
  maxAge: '1y',
  immutable: true
}));

Optimize Cache-Control headers:

# Versioned static assets
Cache-Control: public, max-age=31536000, immutable

# HTML documents
Cache-Control: public, max-age=0, must-revalidate

tip
Use versioned filenames (e.g., app.v123.js) so you can cache aggressively without worrying about stale content. Build tools like Webpack and Vite handle this automatically with content hashing.

Ensure your CDN supports modern protocols:

HTTP/2 or HTTP/3: Multiple resources load in parallel over a single connection
TLS 1.3: Faster connection setup (1 round trip instead of 2)
Brotli compression: 10-20% smaller files than gzip

CDN Best Practices

✅ Use a CDN for all static assets (images, CSS, JS, fonts)
✅ Configure long cache durations (1 year) for versioned assets
✅ Enable Brotli compression and HTTP/2+
✅ Preconnect to your CDN origin from your HTML
✅ Monitor cache hit ratio (aim for >90%)
✅ Consider edge computing (Cloudflare Workers, Vercel Edge) for dynamic content optimization

Real User Monitoring (RUM) Implementation

Lab data (Lighthouse, WebPageTest) is useful for finding issues, but Real User Monitoring (RUM) tells you what your actual users experience. Here’s how to implement RUM for Core Web Vitals.

Why RUM Matters

Diverse conditions: Real users have varying devices, network speeds, and geographic locations
Actual content: Lab tests might not include cookie banners, personalized content, or third-party scripts
User behavior: Real interactions (scrolling, clicking) can reveal layout shifts and input delays that lab tests miss

important
Google uses RUM data (from the Chrome User Experience Report) to determine if your site meets Core Web Vitals thresholds. Your lab scores don’t directly impact rankings—only real user experiences do.

Implementing RUM with web-vitals Library

The web-vitals library (maintained by Google) is a tiny (~2KB) library that accurately measures Core Web Vitals. Install it via npm or load from CDN:

npm install web-vitals

Basic implementation:

import {onCLS, onINP, onLCP} from 'web-vitals';

function sendToAnalytics({name, value, id, rating}) {
  const body = JSON.stringify({
    metric: name,      // 'CLS', 'INP', or 'LCP'
    value: value,      // The metric value
    id: id,            // Unique ID for this page visit
    rating: rating     // 'good', 'needs-improvement', or 'poor'
  });
  
  navigator.sendBeacon('/analytics', body);
}

onCLS(sendToAnalytics);
onINP(sendToAnalytics);
onLCP(sendToAnalytics);

note
The callback fires multiple times as metrics update. Use the id field to deduplicate values in your analytics backend.

Attribution Build for Debugging

The standard web-vitals build tells you what your scores are. The attribution build tells you why they’re bad:

// Import from 'web-vitals/attribution' instead
import {onCLS, onINP, onLCP} from 'web-vitals/attribution';

function sendToAnalytics({name, value, id, rating, attribution}) {
  const data = {
    metric: name,
    value: value,
    rating: rating,
    id: id
  };
  
  // Attribution data helps debug issues
  switch (name) {
    case 'LCP':
      data.element = attribution.element;          // LCP element selector
      data.url = attribution.url;                  // LCP resource URL
      data.ttfb = attribution.timeToFirstByte;     // TTFB timing
      data.renderDelay = attribution.elementRenderDelay;
      break;
      
    case 'INP':
      data.element = attribution.interactionTarget; // Element clicked/tapped
      data.type = attribution.interactionType;      // 'pointer' or 'keyboard'
      data.inputDelay = attribution.inputDelay;
      data.processingTime = attribution.processingDuration;
      data.presentationDelay = attribution.presentationDelay;
      break;
      
    case 'CLS':
      data.element = attribution.largestShiftTarget;
      data.value = attribution.largestShiftValue;
      break;
  }
  
  navigator.sendBeacon('/analytics', JSON.stringify(data));
}

onCLS(sendToAnalytics);
onINP(sendToAnalytics);
onLCP(sendToAnalytics);

This attribution data is gold for debugging. For example:

LCP: If renderDelay is high, you know the image loaded quickly but rendering was blocked (possibly by JavaScript)
INP: If inputDelay is high, the main thread was busy when the user clicked. If processingDuration is high, your event handlers are too slow.
CLS: The largestShiftTarget tells you exactly which element caused the biggest shift.

tip

In production, only send attribution data for “poor” scores to reduce noise and data volume:

onLCP((metric) => {
  if (metric.rating === 'poor') {
    sendDetailedReport(metric); // Send full attribution
  } else {
    sendBasicReport(metric);    // Just send value
  }
});

Analyzing RUM Data

Once you’re collecting data, look for patterns:

Segment by device type: Mobile vs. desktop often have very different performance
Segment by connection type: 4G vs. 5G vs. WiFi
Segment by geography: Users far from your servers will have higher TTFB
Look at the 75th percentile, not the median—that’s what Google uses for Core Web Vitals

bonus
Chrome User Experience Report (CrUX) provides free aggregated RUM data for millions of websites. Check your site at PageSpeed Insights or use the CrUX API for programmatic access.

Putting It All Together

Combine these techniques for maximum impact:

Start with RUM Data: Implement web-vitals library with attribution build to identify problematic pages
Optimize LCP: Use fetchpriority="high" on LCP image, preconnect to CDN origins, configure aggressive caching
Optimize INP: Break up long tasks with scheduler.yield(), batch work, yield strategically in event handlers
Monitor and Iterate: Track improvements in RUM data, set up alerts for regressions

tip
Start small: pick one page with poor Core Web Vitals scores, apply these techniques, measure the impact with RUM data, then expand to other pages.

Conclusion

Advanced Core Web Vitals optimization requires resource prioritization, intelligent task scheduling, strategic CDN usage, and continuous monitoring with real user data.

Key takeaways:

Use fetchpriority="high" on LCP images and preconnect to critical origins
Break up long tasks with scheduler.yield() to keep INP low
Configure CDN with long cache durations and modern protocols (HTTP/2+, Brotli)
Implement RUM with web-vitals library to track real user experiences and debug with attribution data

These techniques bridge the gap between “needs improvement” and “good” scores, translating to faster loading, smoother interactions, and higher engagement. Optimization is ongoing—keep monitoring RUM data, stay informed about new browser APIs, and continuously iterate.