Have a PWA app shell for WebPages

The PWA App shell means having a service worker installed that caches the website assets like JS, CSS, and images locally on the client side (real devices), which will make actual visits and web page performance better for next visits or repeat visits.

This will improve the overall field data, as the users will have fewer HTTP requests and more cached requests, thus getting good performance always thanks to PWA service workers. 

You can go through the details from this article to learn more about building PWA App Shell – https://developer.chrome.com/blog/app-shell?hl=en