Test the pages that matter to a customer
Start with the pages people use to decide and act: the homepage, an important service page, a product or booking page, and the enquiry route. A fast homepage cannot compensate for a difficult checkout.
Use a phone as well as a desktop. Try a first visit, scroll through the content, open the navigation, and interact with the main controls. Note what feels delayed and what moves unexpectedly.
Understand what the metrics describe
Core Web Vitals cover loading, responsiveness, and visual stability through Largest Contentful Paint, Interaction to Next Paint, and Cumulative Layout Shift. A lab test is a controlled diagnostic; real-user data describes observed visits. The two can differ.
Use measurements to locate a problem, then inspect the relevant experience. A large image, delayed script, or moving layout needs a specific fix. Repeating a test until it produces a nicer score does not improve the website.
Make the fix proportionate
Check image dimensions and file sizes, whether lower-page images can load later, and whether fonts or scripts delay useful content. Review third-party widgets with the person who needs them. Removing a script that supports an essential workflow can introduce a different problem.
Reserve space for images and embedded content so the page does not jump as it loads. After a change, check both speed and function. Performance work should leave navigation, forms, and content intact.
Agree on evidence before commissioning work
Ask for the pages and device conditions that will be checked, the issues found, and a record of the changes. Keep before-and-after measurements under comparable conditions. Explain any limits where a supplier cannot control an external service.
Performance is part of website quality alongside readable content, accessibility, and a working enquiry route. Review those together so an improvement in one number does not hide a broken customer journey.
Further reading: web.dev: Web Vitals.