I would like to know how to do this as well. It would be helpful with Change between many UI frameworks.
Hopefully this becomes reality sometime sooner than later: https://trello.com/c/24s6vyxo/55-incremental-loading
Some tools that might help in the meantime:
https://atmospherejs.com/numtel/publicsources
https://atmospherejs.com/numtel/privatesources
https://atmospherejs.com/miro/preloader