PageSpeed Insights vs WebPageTest for a Core Web Vitals fix

PageSpeed Insights vs WebPageTest for a Core Web Vitals fix
A news site was failing CWV on mobile. I tested both, real URLs.

PageSpeed Insights
➕ Field (CrUX) data showed the REAL 28-day user experience, not a lab guess
➖ Lab suggestions were generic ("reduce JS")

WebPageTest
➕ Filmstrip + waterfall pinpointed a render-blocking font and a 1.8s ad-script stall
➖ Steeper learning curve

The case: PSI told us we were failing (LCP 4.1s field). WebPageTest's waterfall showed the exact culprits. We preloaded the font and deferred the ad script. 30 days later, field LCP dropped to 2.3s, the page passed CWV, and the URL group gained +12% clicks as it cleared the threshold.

Best for: PSI to know IF you're failing (field data); WebPageTest to know WHY.
Price-vs-value: Both free. Use them together, in that order.

Pick WebPageTest when PSI flags a fail but won't say why.
Skip optimising on lab scores alone — field data is the one Google ranks on.