Google PageSpeed Insights
Content Plan
This document should cover using PageSpeed Insights for WordPress performance testing.
Planned Content
TL;DR
- How to run a PSI test and interpret results
- Key metrics to focus on (LCP, CLS, FID/INP)
- When to use PSI vs other tools
What is PageSpeed Insights
- Google's web performance analysis tool
- Combines lab data (Lighthouse) with field data (CrUX)
- Mobile vs Desktop scoring differences
- How scores impact SEO and Core Web Vitals
Running a Test
- Navigate to https://pagespeed.web.dev/
- Enter the URL to test
- Understanding mobile vs desktop toggle
- Interpreting the overall score (0-100)
Understanding the Metrics
Core Web Vitals
| Metric | Good | Needs Improvement | Poor |
|---|---|---|---|
| LCP (Largest Contentful Paint) | ≤2.5s | 2.5-4s | >4s |
| FID (First Input Delay) | ≤100ms | 100-300ms | >300ms |
| INP (Interaction to Next Paint) | ≤200ms | 200-500ms | >500ms |
| CLS (Cumulative Layout Shift) | ≤0.1 | 0.1-0.25 | >0.25 |
Other Important Metrics
- First Contentful Paint (FCP)
- Time to First Byte (TTFB)
- Speed Index
- Total Blocking Time (TBT)
Lab Data vs Field Data
- Lab data: Simulated test results (what you see in Lighthouse)
- Field data: Real user measurements from Chrome users (CrUX)
- Why they may differ
- Which to prioritize for different decisions
Common WordPress Issues & Fixes
| PSI Diagnostic | Typical WordPress Cause | Solution |
|---|---|---|
| Reduce unused JavaScript | Theme/plugin bloat | Dequeue unused scripts, use asset cleanup plugin |
| Reduce unused CSS | Theme framework CSS | PurgeCSS, asset optimization plugins |
| Serve images in next-gen formats | JPEG/PNG images | Enable WebP/AVIF conversion |
| Properly size images | Missing srcset | Use responsive images, regenerate thumbnails |
| Eliminate render-blocking resources | Unoptimized CSS/JS loading | Defer JS, inline critical CSS |
| Reduce server response time | Slow hosting/no cache | Enable page caching, optimize database |
Pre-Migration Testing Protocol
- Test homepage, key landing pages, and one blog post
- Document scores and metrics before migration
- Screenshot the full report
- Note any existing issues to address post-migration
Post-Optimization Verification
- Clear all caches before testing
- Run 3 tests and average the results
- Compare against pre-optimization baseline
- Document improvements for client reporting
Limitations
- Lab tests don't account for user geography
- Mobile throttling may not match real devices
- Scores fluctuate between tests (±5 points normal)
- Some recommendations don't apply to all sites