How does the application (app) shell of a Progressive Web App (PWA) improve performance in poor network conditions?

How does the application (app) shell of a Progressive Web App (PWA) improve performance in poor network conditions?

Answer

•  The app shell is a widely accepted standard that allows PWAs to be downloaded directly from a mobile website.
•  The app shell runs in the background, allowing it to pre-load dynamic content for faster performance.
•  The app shell runs key JavaScript files, which can be used in place of dynamic content normally downloaded over a network.
•  The app shell stores (caches) the key user interface elements on the mobile device.