How to use Lighthouse to audit and score page performance

Using Lighthouse Effectively

Lighthouse is an automated auditing tool built into Chrome DevTools. The performance score (0–100) is a weighted average of six lab metrics. LCP and TBT (Total Blocking Time) carry the most weight.

Run Lighthouse from the command line for consistent, headless results:

npx lighthouse https://example.com \
  --preset=desktop \
  --output=html \
  --output-path=./report.html \
  --chrome-flags="--headless"

Lighthouse reports two types of findings:

• Opportunities — actionable fixes with estimated savings (e.g., "Properly size images — save 320KB")

• Diagnostics — contextual data without direct score impact (e.g., DOM size, main thread work)

Score variability is real: a score can fluctuate ±5 points between runs due to CPU and network conditions. Always run at least 3 times and take the median. In CI, use thresholds, not exact scores:

// lighthouserc.js
module.exports = {
  assert: {
    assertions: {
      'categories:performance': ['error', { minScore: 0.85 }],
      'first-contentful-paint': ['warn', { maxNumericValue: 2000 }],
      'largest-contentful-paint': ['error', { maxNumericValue: 2500 }]
    }
  }
};

The most common Lighthouse trap: testing on a fast dev machine over a wired connection. Always use the simulated throttling (Mobile preset = 150ms RTT, 1.6Mbps) to represent median real-world conditions.