Data reads just need to be asynchronous and read at time of request. The standard can't worry about how people might mess up using the feature (not security but performance). A program on my desktop takes time to load and read data. A web app shouldn't be any different given it has the same functionality as the desktop app. The web world needs to stop worrying about performance so much and just work on functionality first. Don't limit functionality because a fear of performance. Of course work on performance but if I can't do it at all, then it's worthless.
Loading more than 5mb on a phone could take a noticeable while, so it's not something you want to do every page load.