How Google Measures Website Performance

When you hear "Google measures my site at 78 on mobile," what's actually happening behind the scenes? Google uses several overlapping tools, each with a different job. Here's how they fit together.

The four tools you should know about

1. Lighthouse (the engine)

Lighthouse is an open-source auditing tool maintained by Google. It runs in a controlled Chrome environment and measures dozens of metrics across Performance, Accessibility, Best Practices, and SEO. It's the same engine that powers:

• Chrome DevTools (Audits tab)
• PageSpeed Insights
• Search Console's Core Web Vitals report (partially)
• Most third-party site testing tools, including RateMySite.io

A Lighthouse run produces a score (0–100) for each category and a detailed breakdown of issues.

2. PageSpeed Insights (the public tool)

PageSpeed Insights (PSI) is Google's public website at pagespeed.web.dev. When you enter a URL, PSI runs a Lighthouse audit on Google's servers with standardized conditions, and combines the results with real-user data from Chrome (see CrUX below).

PSI is the closest thing to an "official" Lighthouse score because it removes variance from your local machine and network.

3. Chrome User Experience Report (CrUX)

CrUX is a dataset of real performance data from millions of opted-in Chrome users. It records actual Core Web Vitals (LCP, INP, CLS) and timing data from real visits to your site.

CrUX data is what Google actually uses for SEO rankings. The Lighthouse score is a lab simulation. CrUX is the real world.

You need a certain amount of traffic for CrUX to have enough data on your URL — small sites might only get site-wide data instead of per-URL data, and brand new sites may have no CrUX data at all.

4. Google Search Console

Search Console has a Core Web Vitals report that shows you which URLs on your site are passing or failing CWV thresholds, grouped by similar pages. It's based on CrUX data and is the most useful tool for prioritizing where to focus your optimization work.

How they fit together

                ┌─────────────┐
                │  Your Site  │
                └──────┬──────┘
                       │
   ┌───────────────────┼───────────────────┐
   │                   │                   │
   ▼                   ▼                   ▼
┌─────────┐      ┌──────────┐       ┌──────────┐
│  Lab    │      │  Real    │       │  Real    │
│  test   │      │ Chrome   │       │  Google  │
│ (LH)    │      │ users    │       │  bot     │
└────┬────┘      └────┬─────┘       └────┬─────┘
     │                │                  │
     ▼                ▼                  ▼
┌─────────┐      ┌──────────┐       ┌──────────┐
│ PSI lab │      │  CrUX    │       │ Indexing │
│ score   │      │  data    │       │ & rank   │
└─────────┘      └────┬─────┘       └────┬─────┘
                      │                  │
                      └─────┬────────────┘
                            ▼
                   ┌────────────────┐
                   │ SEO ranking    │
                   │ signal         │
                   └────────────────┘

The simple version:

• Lighthouse / PSI lab score = diagnostic tool to find and fix issues
• CrUX data = the real number Google uses for ranking
• Search Console = the dashboard showing your CrUX results

What Lighthouse actually checks

Lighthouse runs four categories:

1. Performance — speed and Core Web Vitals
2. Accessibility — WCAG compliance, contrast, semantic HTML
3. Best Practices — security, modern APIs, console errors
4. SEO — meta tags, crawlability, mobile-friendliness

Each gets a 0–100 score. The Performance score is a weighted average of timing metrics — see what PageSpeed Insights actually checks for the full breakdown.

How the score is calculated

The performance score uses a log-normal distribution scaled against real-world Chrome data. That means small improvements at the low end (going from 30 to 40) take less work than improvements at the high end (90 to 95).

The score is calculated from:

• Largest Contentful Paint (25%)
• Total Blocking Time (30%)
• Cumulative Layout Shift (25%)
• First Contentful Paint (10%)
• Speed Index (10%)

INP is reported by Lighthouse but isn't currently part of the score weighting (it can only be measured in field data, not lab).

What Google uses for ranking

The page experience update (rolled out 2021, refined since) made Core Web Vitals a real ranking factor. Specifically:

• Google looks at CrUX field data, not lab scores.
• A URL "passes" if all three Core Web Vitals are in the green for 75% of page loads.
• Passing CWV is a tiebreaker — it won't save bad content, but it'll boost good content above competitors.
• Mobile CWV matters more than desktop because of mobile-first indexing.

A perfect 100 Lighthouse score with no CrUX traffic = no SEO benefit. A 65 lab score with great real-user CWV = full SEO benefit.

What this means for you

1. Use Lighthouse for diagnosis. When something's slow, Lighthouse tells you what and why.
2. Use CrUX for verification. After fixing, check your CrUX data (in PSI or Search Console) to confirm real users see the improvement. CrUX data updates with a 28-day rolling window, so changes take time to show up.
3. Focus on the metrics Google actually ranks on. That's LCP, INP, and CLS in the green.
4. Don't chase 100. A 90 lab score with passing real-user CWV is better than a 100 lab score nobody actually measures.

Try it yourself

Run a free audit on RateMySite — we show you both the lab score (Lighthouse) and a letter grade that reflects what real users would experience. For raw PageSpeed numbers, pagespeed.web.dev is the authoritative source.

For more reading, see what PageSpeed Insights actually checks and Core Web Vitals explained for beginners.