On the – non Beta – Desktop version, it works as a single folder file (pretty sizeable, +150mb) that you can host anywhere, and renders the prototype within an iframe.
The team at Farfetch did a custom component to do something similar to what you mention (if I understood you well): https://youtu.be/LEMGL2NH0r8?t=2760
I think someone else said there are other optimizations that happen when you shop production code.
Aside from that, they use the app literally their entire workday, so spending 5 seconds loading the thing isn’t going to hurt anyone.
It can be slightly hacked around to add user authentication & analytics, so if you are handling a small amount of users its a really efficient way to iterate fast.
Of course the initial users will experience a slow product, but the idea is to discover faster what a useful product looks like without investing development resources.
But let us be realistic. I just loaded www.amazon.com - 17.8 MB transferred when it finally stopped. Arguably one of the most successful websites in the world.
A 5MB asset size for most sites is perfectly acceptable.