Real User Metrics are driven by real user interaction. The speed at which a single page loads from a new browser (GTMetrix, Lighthouse) is useful for identifying opportunities for improvement during development, but in order to improve SERP (Search Engine Result Pages) you need to optimize real data.
A key breakthrough in effecting real metrics is the development of the Service Worker, which adds the ability to cache assets more effectively on the client device.
Client side optimizations like the service worker cannot be measured by synthetic or singular user interactions (such as GTMetrix and Lighthouse), but they can result in dramatic web performance increases.
Service workers are powerful and absolutely worth mastering. They let you deliver an entirely new level of experience to your users. Your site can load instantly. It can work offline. It can be installed as a native app and feel every bit as polished—but with the reach and freedom of the web. But service workers are unlike anything most of us web devs are used to. They come with a steep learning curve and a handful of snags you've got to watch out for. Source - Google®
Service Workers and other client side optimization techniques cannot be accurately measured with a single page load. Unlike server side optimizations like image resizing, faster hosting providers and improved CDN services, client side optimizations work alongside the browser throughout the user experience. Advanced client side platforms such as Edgemesh can implement client side load balancing, adaptive pre-caching of website assets and even implement the ability to block certain 3rd party scripts from loading based on network, geography or device type.