I used to need to use a large web app like that at work. You'd get a login page. Submitting your login caused the app to load, which was many MB. Once that was all downloaded, the page would finally display, and you could do what you needed to. (In our case, you usually only needed to look something up quickly and be done, which is a bit different than your case.)
But the real kicker is... the company VPN was slow. Really slow. In fact, most of the time, as soon as the app finally loaded, the first thing it did was pop up a dialog telling you your session had expired due to inactivity and send you back to the login page to start over from scratch.
I sure missed the simple, small, usable, multi page site we used before the "upgrade" to the single page app competitor! (On top of that, the old one worked fine on mobile. The new one, not at all.)