Why Your Perfect Lighthouse Score Doesn't Mean Your Site is Fast

Great Lighthouse scores, but users still complain your site is slow? You're not alone. Here's a shocking reality check: Google's research found that 50% of websites with perfect Lighthouse scores still fail Core Web Vitals when measured with real user data. Half! That means half the developers celebrating perfect synthetic scores are actually delivering poor performance to their real users. The Problem with Synthetic Testing Lighthouse and PageSpeed Insights run in perfect, controlled environments. They simulate an "85th percentile user" — but their simulation is usually wrong for your specific audience. Real-world variables synthetic tests miss: Network packet loss and latency spikes Device thermal throttling during extended use Background apps competing for resources Browser extensions affecting performance User behavior patterns (scrolling, multitasking) Third-party vendor outages Your developer-focused B2B app has completely different users than a consumer social platform, but Lighthouse treats them the same. What Real User Monitoring Actually Shows You Real User Monitoring (RUM) collects performance data from actual user sessions as they happen. Instead of guessing what your users experience, RUM shows you the reality. RUM catches problems synthetic testing misses:

Jun 5, 2025 - 17:40

Why Your Perfect Lighthouse Score Doesn't Mean Your Site is Fast

Great Lighthouse scores, but users still complain your site is slow? You're not alone.

Here's a shocking reality check: Google's research found that 50% of websites with perfect Lighthouse scores still fail Core Web Vitals when measured with real user data. Half!

That means half the developers celebrating perfect synthetic scores are actually delivering poor performance to their real users.

The Problem with Synthetic Testing

Lighthouse and PageSpeed Insights run in perfect, controlled environments. They simulate an "85th percentile user" — but their simulation is usually wrong for your specific audience.

Real-world variables synthetic tests miss:

Network packet loss and latency spikes
Device thermal throttling during extended use
Background apps competing for resources
Browser extensions affecting performance
User behavior patterns (scrolling, multitasking)
Third-party vendor outages

Your developer-focused B2B app has completely different users than a consumer social platform, but Lighthouse treats them the same.

What Real User Monitoring Actually Shows You

Real User Monitoring (RUM) collects performance data from actual user sessions as they happen. Instead of guessing what your users experience, RUM shows you the reality.

RUM catches problems synthetic testing misses: