Since you say it's all JSON APIs, it should be trivial to build something on the server to server-render the initial page with content and serve that up while the rest of the application tries to download itself into the browser.
This approach was a commercial failure on the native platform level (those Office-like app suites with network install of components each time you needed them). It was also a commercial failure on the "build my desktop from scratch when I boot up" environments.
It yet another tick in the "does not belong in a browser" list.
Perhaps Single Page Applications need to run inside a window where there's no browser navigation bar, no url bar. Stripped down so far it's not actually a functioning browser. That feels very much like a native GUI library at that point. Except for that ludicrous notion of the app needing to be installed each time the chrome/window is launched.
I can't see how "install the app each time you land on this website" is competitive against native's "install this app once, and run many times" approach.