← All Blogs

Progressive Loading Patterns: Lazy, Eager & Priority

January 15, 2025 • WebCore Team • Performance + Demo

Strategic loading patterns can dramatically improve your Core Web Vitals. Learn when to prioritize hero content, defer non-critical assets, and use modern browser hints for optimal performance.

Based on recent 2024-2025 studies, we'll explore fetchpriority, lazy loading best practices, and LCP optimization through interactive demos.

Loading Strategy Comparison

Compare how different loading strategies affect performance metrics and user experience

Default Loading
Browser decides priority
Standard browser behavior
Optimized Loading
Strategic prioritization
Eager hero + lazy below-fold
Priority Hints
Modern fetchpriority
fetchpriority="high"

Performance Metrics

Largest Contentful Paint (LCP) —
First Contentful Paint (FCP) —
Cumulative Layout Shift (CLS) —

Notice how strategic loading patterns reduce LCP times and improve perceived performance through proper asset prioritization.

The Three Pillars of Progressive Loading

1. Eager Loading: Prioritize Critical Content

Eager loading ensures critical above-the-fold content loads immediately. According to 2024 HTTP Archive data, 82% of pages with good LCP scores properly eager-load their hero images.

<img src="hero.webp" loading="eager" fetchpriority="high" alt="Hero image" />

Key insight: Never lazy-load LCP elements. Recent studies show this mistake affects over 730,000 websites and can delay LCP by 200-500ms.

2. Lazy Loading: Defer Non-Critical Assets

Lazy loading defers below-the-fold images until they're needed, reducing initial bandwidth usage. Modern native lazy loading has 94% browser support and uses optimized intersection observers.

<img src="below-fold.webp" loading="lazy" alt="Below fold image" />

Performance impact: Proper lazy loading can improve initial page load by 15-30% according to Chrome's 2024 performance research.

3. Priority Hints: Fine-Tune Resource Loading

Priority hints via the fetchpriority attribute give developers granular control over resource loading order. Supported in Chrome 101+ and Edge 101+.

<img src="lcp-image.webp" fetchpriority="high" alt="LCP image" />
<img src="secondary.webp" fetchpriority="low" alt="Secondary" />

Real-world results: Companies like Etsy report 4% LCP improvements after implementing fetchpriority on hero images.

Core Web Vitals Impact

Largest Contentful Paint (LCP) Optimization

  • Hero image strategy: Use fetchpriority="high" + eager loading
  • Resource load delay: Minimize by preloading LCP elements
  • Format optimization: WebP/AVIF can reduce LCP by 15-25%
  • Responsive images: Prevent oversized downloads with proper srcset

Cumulative Layout Shift (CLS) Prevention

  • Reserve space: Always include width and height attributes
  • Aspect ratio CSS: Use aspect-ratio for flexible layouts
  • Placeholder strategy: Low-quality image placeholders prevent shifts
  • Progressive enhancement: Load high-quality versions without layout changes

Advanced Techniques

Two-Stage Loading

Advanced progressive loading uses low-quality placeholders that swap to high-quality versions. This technique can improve perceived performance by 40-60%.

// Stage 1: Load LQIP (Low Quality Image Placeholder)
<img src="hero-lqip.webp" onload="loadHighQuality(this)" />

// Stage 2: Progressive enhancement
function loadHighQuality(img) {
  const highRes = new Image();
  highRes.onload = () => img.src = highRes.src;
  highRes.src = "hero-hq.webp";
}

Intersection Observer Optimization

For custom lazy loading implementations, optimize intersection observer settings for better performance:

const imageObserver = new IntersectionObserver((entries) => {
  entries.forEach(entry => {
    if (entry.isIntersecting) {
      loadImage(entry.target);
      imageObserver.unobserve(entry.target);
    }
  });
}, {
  rootMargin: '50px' // Load 50px before entering viewport
});

Browser Support & Fallbacks

Current Browser Support (2024-2025)

  • Native lazy loading: 94% global support
  • fetchpriority: 73% support (Chrome, Edge, Safari)
  • Intersection Observer: 95% support
  • WebP format: 96% support
  • AVIF format: 77% support

Implementation Checklist

Progressive Loading Audit

Research Sources & Further Reading

  1. NitroPack. "10+ New Optimizations For Your 2025 Core Web Vitals Strategy." December 2024. View Article
  2. HTTP Archive. "Performance Mistakes - An HTTP Archive Deep Dive." 2024. View Report
  3. Google Chrome. "Priority Hints and fetchpriority." February 2024. View Guide
  4. Kevin Farrugia. "Fetch Priority and optimizing LCP." January 2023. View Study