Why do menus in the game start lagging if my roommate is downloading a game?

[Kirutsuki]

It's because it's an online game....

I kind of know all of this. But, I play other online videogames and menus, item management etc. work perfectly fine even with poor connection.
The ONLY reason it works this way with 14 seems to be that nothing is stored locally in any capacity. So all the items in your inventory is stored in a server somewhere.

I understand the reason why you wouldn't want a local data stored on the players PC in an MMO, and I undestand all these ways of which the connection could be improved, but fundamentally there's some really archaic code underneath where UI's and item handling is network dependant.