This is a solved problem via service workers. They can cache all the app's content and you can choose to cache whatever you like including api responses from any service. They could also be configured to serve cache first and replace the content in background. At this point if the app is slow to start, it's 100% on devs/architecture. The only problem is if you exceed the quota for cache, which is varied by browser/platform, you risk the browser just nuking all your cache/localstorage/indexeddb.