I don't know the details about which parts of the simulation are run locally and server-side, but the fact that your cities are "always on" (i.e. other players' cities can trade and visit your city even if you're not playing) tells me that a good portion of the resources/traffic simulation is run on Maxis servers.
It may be not be a valid reason for you, but Sim City ran significantly faster than Civilization V on my machine. To me, they both perform a massive number of simultaneous simulations, and Sim City's performance is near instantaneous.
That said, there's one huge factor none of the beta players can take into account when judging Sim City's performance: no one has been able to build massive, multiple-city metropolises, as play time was limited to hour-long sessions.
It may be not be a valid reason for you, but Sim City ran significantly faster than Civilization V on my machine. To me, they both perform a massive number of simultaneous simulations, and Sim City's performance is near instantaneous.
That said, there's one huge factor none of the beta players can take into account when judging Sim City's performance: no one has been able to build massive, multiple-city metropolises, as play time was limited to hour-long sessions.