Measure the experience people receive
A performance score is a diagnostic aid, not the entire experience. Core Web Vitals cover loading through Largest Contentful Paint, responsiveness through Interaction to Next Paint and visual stability through Cumulative Layout Shift.
Lab measurements help reproduce a problem under controlled conditions. Field measurements show what actual visitors experience across devices and networks. A new or low-traffic site may not yet have enough field data for a reliable view.
Find the expensive part before changing it
Check the main content, images, fonts, scripts and third-party integrations. Large background videos and decorative libraries can compete with the information a visitor came to read. A layout that moves as fonts or images load can also make an otherwise fast page frustrating.
Prioritise changes around the busiest and most important journeys. A contact form that responds clearly to a tap can matter more than a minor improvement to an unused animation. Set image dimensions and avoid making essential content depend on an entrance effect.
Keep checking after launch
Measure representative pages on a mobile viewport with a constrained network, then test with real devices when available. Recheck when you add analytics, a booking widget, a new media format or another external script.
Agree a performance budget and keep a small set of repeatable checks in the release process. Performance is ongoing work: content and integrations change even when the layout stays the same.