Rock, Paper, Shotgun has quite the exclusive today, reporting from a SimCity developer who reached out to them that Maxis don't actually offload a large portion of their game's calculations to their servers. The Maxis insider has said that the server isn't actually handling these calculations for non-social aspects of the game, and that engineering a single-player mode would not require that much effort.
RPS have verified that the Maxis insider works on the game, and has first-hand knowledge of how SimCity works. This insider has also made is clear that his repeated claims of server-side calculations is at odds with the reality of the project he worked on, where he explains:
The servers are not handling any of the computation done to simulate the city you are playing. They are still acting as servers, doing some amount of computation to route messages of various types between both players and cities. As well, they're doing cloud storage of save games, interfacing with Origin, and all of that. But for the game itself? No, they're not doing anything. I have no idea why they're claiming otherwise. It's possible that Bradshaw misunderstood or was misinformed, but otherwise I'm clueless.
There are many gamers out there who can run the game, even if it's just for 20-30 minutes, offline, without requiring the servers. The next question is, if those gamers are able to play with the servers in this state, what are the servers actually required to do? Sure, they do a lot of stuff in the game, but are they actually required?
Because of the way Glassbox was designed, simulation data had to go through a different pathway. The game would regularly pass updates to the server, and then the server would stick those messages in a huge queue along with the messages from everyone else playing. The server pulls messages off the queue, farms them out to other servers to be processed and then those servers send you a package of updates back. The amount of time it could take for you to get a server update responding to something you've just done in the game could be as long as a few minutes. This is why they disabled Cheetah mode, by the way, to reduce by half the number of updates coming into the queue.