Understanding CrUX vs Lab Data: Why Real Users Are Slower

Reading time: 13 minutes

Your Scan report shows Lab LCP: 2.1s (Good) but Real User LCP: 4.2s (Poor). That’s a 2-second gap. And the real user score is what Google uses for rankings. Here’s why the difference exists and how to close it.

TLDR

Lab data comes from simulated tests on fast devices with perfect connections. CrUX shows real visitor performance across 28 days of actual Chrome users. Typical gap: lab shows 2.1 seconds, CrUX shows 4.2 seconds. twice as slow in the real world. The difference comes from slower devices, variable networks, browser extensions, and cache failures. Google ranks based on CrUX, not lab tests. Close the gap by optimizing for slow devices and networks, setting cache headers, and deferring third-party scripts.

The Two Types of Performance Data

Lab Data (Lighthouse/PageSpeed Insights):

• Simulated test from Google’s servers
• Consistent testing environment
• Fast connection, powerful device
• Empty cache (first-time visitor simulation)
• No browser extensions

CrUX (Chrome User Experience Report) - Real User Data:

• Actual performance from real visitors
• 28-day rolling average
• Mix of devices (iPhone 11, Samsung A52, etc.)
• Mix of connections (5G, 4G LTE, slow 3G, rural broadband)
• Real browser extensions (ad blockers, etc.)
• Returning visitors with cached resources

Typical Scan report:

Lab Performance (Simulated):
LCP: 2.1s (Good)
CLS: 0.02 (Good)
INP: 150ms (Good)
Speed Score: 88/100

Real User Performance (Chrome UX Report):
LCP: 4.2s (Poor) - 75th percentile
CLS: 0.12 (Needs Work) - 75th percentile
INP: 280ms (Needs Work) - 75th percentile

The gap: 2.1s lab vs 4.2s real users = 100% slower in real world

Why Real Users Are Slower

Reason 1: Device Performance (30% of gap)

Lab test device:

• Desktop-class CPU
• 16GB RAM
• Fast GPU

Real user devices (your actual customers):

• iPhone 11 (2019): 4GB RAM, slower A13 chip
• Samsung Galaxy A52 (mid-range): 6GB RAM, mid-tier CPU
• Budget Android phones: 3-4GB RAM, entry-level CPU

Impact:

• JavaScript execution: 2-3x slower on real devices
• Image decoding: 1.5-2x slower
• CSS parsing: 1.5x slower

Example: Your homepage has 300KB JavaScript.

• Lab (powerful CPU): Parses in 200ms
• iPhone 11: Parses in 450ms
• Budget Android: Parses in 600ms

Gap: +250-400ms just from device CPU difference

Reason 2: Network Conditions (40% of gap)

Lab test connection:

• Simulated “4G LTE” (10 Mbps)
• No packet loss
• Consistent latency

Real user connections:

• 4G LTE (actual): 3-8 Mbps (congested towers)
• Rural broadband: 2-5 Mbps
• Slow 3G: 1-2 Mbps (still 15% of mobile users)
• Hotel/airport WiFi: Variable, often throttled

Impact:

Your hero image: 500KB

• Lab (10 Mbps): Downloads in 0.4 seconds
• Real 4G LTE (5 Mbps avg): Downloads in 0.8 seconds
• Slow 3G (1 Mbps): Downloads in 4 seconds

Gap: +0.4-3.6 seconds just from network variance

Reason 3: Browser Extensions and Ad Blockers (15% of gap)

Lab test browser:

• Clean Chrome install
• No extensions
• No ad blockers

Real user browsers:

• Ad blockers (40% of users): Block ads but add processing time
• Password managers: Inject scripts into forms
• Accessibility tools: Add extra DOM parsing
• VPNs: Add latency to all requests

Impact:

• Each extension: +50-150ms processing overhead
• Ad blocker: Blocks ads (good) but scans DOM for ad elements (+100-200ms)
• Total: +200-500ms from extensions

Reason 4: Cache Behavior (10% of gap)

Lab test:

• Empty cache (simulates first-time visitor)
• But only ONE test run with empty cache

Real users (CrUX):

• 60% are returning visitors (should have cache)
• But cache often fails:
  • Cache-Control headers misconfigured
  • Assets change URLs (cache busting)
  • Users clear cache
  • Mobile browsers aggressive cache clearing (low storage)

Paradox: Lab tests empty cache, real users SHOULD be faster (cached assets), but often aren’t because of cache misconfigurations.

Reason 5: Peak Traffic vs Off-Peak (5% of gap)

Lab test:

• Runs any time of day
• Server not under load

Real users (CrUX):

• Includes peak traffic periods (lunch, evenings)
• Server may be slower under load
• Third-party scripts (analytics, ads) may be slower
• CDN edge nodes may be busier

Impact:

• Server response time: +50-200ms during peak
• Third-party scripts: +100-300ms during peak

The 75th Percentile Explained

CrUX reports 75th percentile, not average.

What this means:

If your CrUX LCP is 4.2s:

• 75% of users experience LCP of 4.2s or faster
• 25% of users experience LCP SLOWER than 4.2s (4.5s, 5s, 6s+)

Why 75th percentile?

Google uses this because:

• It represents typical experience (not best-case)
• It filters out extreme outliers (someone on dial-up)
• It’s the threshold for Google ranking boost

Percentile breakdown example:

Percentile	LCP Time	What This Means
25th (fast)	2.8s	Your fastest 25% of users
50th (median)	3.5s	Typical user experience
75th (CrUX)	4.2s	This is your CrUX score
90th (slow)	5.8s	Your slowest 10-25%
95th (very slow)	7.2s	Extreme cases (slow 3G, old devices)

Google’s ranking requirement: 75th percentile must be “Good” (LCP < 2.5s).

Your situation: 75th percentile: 4.2s = “Poor” → no ranking boost, likely ranking penalty.

How to Close the Gap

Fix 1: Optimize for Slow Devices (Target: -0.5-1s)

Reduce JavaScript:

Audit script size:

# Check total JavaScript size
Open Chrome DevTools → Network → Filter: JS → Reload

Target: Under 300KB total JavaScript (gzipped).

Quick wins:

• Remove unused libraries (old jQuery, etc.)
• Use native JavaScript instead of heavy frameworks
• Defer non-critical scripts
• Code split (load only what page needs)

Expected improvement:

• Lab: 2.1s → 2.0s (minimal change)
• CrUX: 4.2s → 3.5s (slower devices benefit more)

Fix 2: Optimize for Slow Networks (Target: -0.5-1.5s)

Compress images aggressively:

Target file sizes for real users:

• Hero image: Under 100KB (not 200KB)
• Service photos: Under 80KB (not 150KB)
• Thumbnails: Under 30KB (not 50KB)

Use next-gen formats:

• WebP (30% smaller than JPEG)
• AVIF (50% smaller than JPEG, but limited support)

Implementation:

<picture>
  <source srcset="hero.avif" type="image/avif">
  <source srcset="hero.webp" type="image/webp">
  <img src="hero.jpg" alt="Fallback">
</picture>

Expected improvement:

• Lab: 2.1s → 1.9s
• CrUX: 4.2s → 3.0s (slow networks benefit significantly)

Fix 3: Improve Cache Strategy (Target: -0.3-0.8s)

Problem: Returning visitors aren’t getting cache benefits.

Fix: Set aggressive cache headers

# In your .htaccess or server config:
<filesMatch ".(jpg|jpeg|png|gif|webp|css|js|woff2)$">
  Header set Cache-Control "max-age=31536000, public"
</filesMatch>

What this does:

• Tells browsers to cache images/CSS/JS for 1 year
• Returning visitors load from cache (near-instant)

Use cache busting for updates:

style.css?v=1.2.3

When you update CSS, increment version.

Expected improvement:

• Lab: No change (doesn’t test cache)
• CrUX: 4.2s → 3.6s (returning visitors much faster)

Fix 4: Reduce Third-Party Script Impact (Target: -0.2-0.5s)

Audit third-party scripts:

Open your site → DevTools → Network → Reload

Look for external domains:

• google-analytics.com
• facebook.net
• doubleclick.net
• chatwidget.com

Each third-party request:

• DNS lookup: +20-100ms
• Connection: +50-200ms
• Download: +100-500ms

Optimization:

Defer analytics:

<script>
  window.addEventListener('load', function() {
    // Load analytics AFTER page loads
    var script = document.createElement('script');
    script.src = 'https://www.google-analytics.com/analytics.js';
    document.body.appendChild(script);
  });
</script>

Self-host where possible:

• Google Fonts → Download and self-host
• Icon libraries → Inline critical icons as SVG

Expected improvement:

• Lab: 2.1s → 1.8s
• CrUX: 4.2s → 3.4s

Fix 5: Server Response Time (Target: -0.1-0.3s)

Check Time to First Byte (TTFB):

Scan report shows:

TTFB: 650ms (Needs Work)

Target: Under 200ms

Causes of slow TTFB:

• Slow web hosting (shared hosting under load)
• No server caching
• Slow database queries
• No CDN

Fixes:

Quick: Enable server caching

• WordPress: Install W3 Total Cache or WP Rocket
• Shopify: Built-in (already optimized)

Medium: Upgrade hosting

• Move from shared hosting ($5/month) to managed WordPress ($25/month)
• Or use Cloudflare (free tier) as CDN + cache

Expected improvement:

• TTFB: 650ms → 200ms
• Lab: 2.1s → 1.6s
• CrUX: 4.2s → 3.2s

Testing Like Real Users

Chrome DevTools Slow 3G Simulation

1. Open Chrome DevTools (F12)
2. Network tab → Throttling dropdown → “Slow 3G”
3. Device toolbar → Select “Moto G4” (mid-range 2016 Android)
4. Reload page

Experience what your slowest 25% of users experience.

Target: Page should load in under 5 seconds even on Slow 3G + old device.

WebPageTest.org Real Device Testing

https://www.webpagetest.org/

Settings:

• Location: Choose city near your customers
• Device: “Moto G4” or “iPhone 11”
• Connection: “4G LTE” or “3G”

Run test → See filmstrip of loading

You’ll visually see when LCP occurs on real device.

Compare to your CrUX 75th percentile:

• WebPageTest Moto G4 on 4G: 4.0s LCP
• CrUX 75th percentile: 4.2s LCP
• Consistent (validates CrUX data)

Monitoring CrUX Improvements

Important: CrUX updates monthly with 28-day rolling window.

Timeline:

Week 1: Make fixes

• Optimize images, defer scripts, improve caching

Week 2: Verify lab improvement

• Run PageSpeed Insights
• Lab score should improve immediately
• PageSpeed: 88 → 94

Week 3-4: Wait for CrUX

• CrUX still shows old data (28-day rolling average)

Week 5-8: CrUX starts updating

• New user data gradually replaces old data

Week 9+: Full CrUX update

• CrUX now reflects your improvements
• Check Google Search Console

Google Search Console tracking:

1. Search Console → Experience → Core Web Vitals
2. View “Mobile” report
3. Track “Good URLs” count over time

Target progression:

• Month 1: 20% Good URLs
• Month 2: 45% Good URLs (improvements kicking in)
• Month 3: 75% Good URLs (majority passing)
• Month 4: 90%+ Good URLs

When Lab and CrUX Align

Ideal scenario:

Lab Performance:
LCP: 1.8s (Good)

Real User Performance (CrUX):
LCP: 2.3s (Good)

Gap: 0.5 seconds (acceptable)

This means:

• Your optimizations work on real devices
• Real user networks aren’t bottleneck
• Cache strategy is working
• Server performs well under real load

How to achieve this:

1. Test on real mid-range devices (not just lab)
2. Test on slow connections (3G, throttled 4G)
3. Optimize for worst-case (slow device + slow network)
4. Monitor CrUX monthly to verify real-world improvement

Common Mistakes

Mistake 1: Only optimizing for lab score

“My PageSpeed score is 95! Why is CrUX still poor?”

Problem: Lab test doesn’t reflect real devices/networks.

Fix: Test on Slow 3G + Moto G4. Optimize until that passes.

Mistake 2: Expecting instant CrUX updates

“I fixed everything last week but CrUX still shows poor!”

Problem: CrUX is 28-day rolling average.

Fix: Wait 4-6 weeks for full update.

Mistake 3: Ignoring the 75th percentile

“My average LCP is 2.8s (good), why does CrUX show 4.2s (poor)?”

Problem: CrUX uses 75th percentile, not average.

Fix: Optimize for your slowest 25% (who pull the 75th percentile up).

Mistake 4: Forgetting about cache

“Why are returning visitors still slow? They should have cached assets.”

Problem: Cache headers not set properly.

Fix: Set Cache-Control: max-age=31536000 for static assets.

Advanced: Geographic CrUX Data

CrUX can vary by location:

U.S. visitors:

• Good broadband/4G coverage
• Newer devices
• CrUX LCP: 2.8s

Rural areas:

• Slower broadband
• Older devices
• CrUX LCP: 4.5s

If your business serves specific geography:

Google Search Console → Core Web Vitals → Filter by page group

Identify which pages/regions are slowest.

Using Your Scan Report

Your Scan report shows both lab and CrUX:

Lab Performance (What Google tests):
LCP: 2.1s | CLS: 0.02 | Speed Index: 2.8s
Score: 88/100

Real User Performance (What customers experience):
LCP: 4.2s (75th percentile) | CLS: 0.12 | INP: 280ms
Status: Poor (needs immediate improvement)

Prioritization:

1. If lab is poor, fix lab first

   • Lab poor = fundamental issues (huge images, blocking scripts)
   • Fix these before worrying about CrUX

2. If lab is good but CrUX is poor, optimize for real devices

   • Test on slow connections
   • Test on mid-range devices
   • Improve caching
   • Reduce third-party scripts

Your case (lab good, CrUX poor): → Optimize for real-world conditions (slow devices, slow networks)

Next Steps

This Week:

1. Check your Scan report for Lab vs CrUX gap
2. Test your site on Slow 3G + Moto G4 (Chrome DevTools)
3. Identify biggest bottleneck (images? JavaScript? TTFB?)

This Month:

1. Compress images to under 100KB each
2. Defer third-party scripts to load after page
3. Set cache headers for static assets
4. Re-run Scan to see lab improvement

This Quarter:

1. Monitor Google Search Console Core Web Vitals weekly
2. Watch for CrUX improvement (weeks 4-8 after fixes)
3. Target: Close gap to under 1 second between lab and CrUX
4. Goal: 75th percentile CrUX under 2.5s (Good)

→ Related: Core Web Vitals Explained | Mobile Performance