Many people are familiar with the two. Although when it comes down to PageSpeed Insights vs WebPageTest. However, which one should you use, and when do you use it? PageSpeed Insights (PSI) for Google-aligned Core Web Vitals with real-user Field data (CrUX). Use WebPageTest (WPT) when you need a forensic Lab analysis. Waterfalls, filmstrips, request priority, and fine-grained test profiles, to pinpoint what blocks rendering.
Key differences at a glance
| Feature | PageSpeed Insights (PSI) | WebPageTest (WPT) |
|---|---|---|
| Core Web Vitals focus | LCP / INP / CLS (Field + Lab view) | Vitals in traces + filmstrip context |
| Field (real-user) data | Yes (CrUX) | Lab only (test agents) |
| Waterfalls/filmstrip | Basic | Advanced + visual filmstrip |
| Profiles & throttling | Standard Lighthouse settings | Highly customisable devices/locations |
| Best for | Google’s perspective & quick decisions | Forensics & render-path tuning |
When to use PageSpeed Insights
Start with PSI to understand the reality your users face. The Field panel (CrUX) shows your 75th-percentile LCP/INP/CLS; the Lab panel (Lighthouse) estimates where you’ll improve. Use PSI when you need to:
- Confirm whether real users pass Core Web Vitals.
- Get a clear “what to fix first” via Opportunities.
- Compare URL vs Origin performance and track changes after it “deploys.”
WebPageTest proof (Desktop · London): LCP 0.605s, CLS 0, TBT 0.037s
https://www.webpagetest.org/
When to use WebPageTest
WebPageTest is a powerful tool for understanding the why. It reveals blocking requests, priority inversions, and layout/rendering behaviour you can’t see from a single Lighthouse run. Use it when you need:
- Filmstrips to visualise render start, LCP element reveal, and CLS jumps.
- Waterfalls showing DNS/TLS/TTFB, caching, and protocol hints (e.g., HTTP/2 prioritisation).
- Custom profiles: test locations, devices, bandwidth/latency, repeat views, and script steps.
FAQs
Should I rely on the PSI score alone?
No. Treat the score as a headline. Always check the field data (CrUX) and your Core Web Vitals status.
When is WebPageTest essential?
When you’re chasing down poor LCP/TBT, render-blocking, or third-party scripts. The waterfall + filmstrip makes bottlenecks obvious.
Which tool helps SEO more?
Neither directly. Hitting green Core Web Vitals helps users and SEO. We aim for stable green results rather than promising perfect 100/100.
Making PSI and WebPageTest agree (workflow)
- Identify the failing metric in PSI Field. Is it LCP, INP, or CLS? Note the percentile values and device (mobile first).
- Open WPT. Run a test with the closest device/location to your audience. Enable filmstrip and capture waterfalls.
- Fix order & priority: set correct resource priorities (preload hero image & fonts), inline critical CSS, defer non-critical JS, and compress/resize hero media. Aim for your LCP element to be renderable ASAP.
- Trim main-thread work: break up long tasks (<50ms chunks), lazy-hydrate components, and delay 3rd-party tags until interaction to improve INP.
- Eliminate layout shifts: reserve static space for images/ads/embeds, set width/height attributes, and use font preloading with font-display to avoid CLS.
- Re-test in WPT to validate render/TTFB/LCP timing improvements; then re-test PSI to confirm a Field pass.
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**PSI is a testing tool, not an endorsement.**
Troubleshooting differences
- PSI worse than WPT: your real users are slower than the lab profile, or UI differs (consent banners, A/B scripts). Optimise for Field first.
- WPT worse than PSI: your test profile is harsher (slower CPU/network) or the site depends on CDN/geography. Align test settings with user reality.
- Scores vary every run: Performance is stochastic. It will run 3–5 tests, so look at medians and P75, don’t use a single number.
Best practices that help both tools
- Serve smaller, earlier LCP elements: compress/resize hero image or poster frame; avoid autoplaying heavy media; use
fetchpriority/rel=preloadcorrectly. - Reduce JS cost: remove unused scripts, split by route, and keep the main thread free for user input to improve INP.
- Stabilise layout: fixed dimensions for media, avoid layout-thrashing scripts, and load fonts predictably to keep CLS < 0.1.
FAQ
Is WebPageTest better than PSI? Neither is “better”, as they answer different questions. PSI will show you if users pass CWV; WPT shows what needs to be altered to make that happen.
Do I need both? Yes. Use PSI to set the goal (Field pass), WPT to find and verify the fixes.
Next: See GTmetrix vs PSI, Pingdom vs PSI, or return to the PSI guide.