This is still true today - while most computers could handle the graphics engine behind it, the simulation engine would still eat your machine for lunch.
Moving the city simulation server-side certainly has its justifications - it opens up the field to more players with weaker machines.
That being said, it's hard to imagine that, given the ginormous increase in CPU performance between SimCity 4 and now, that they couldn't have made a simulation engine fast enough (and optimized enough) to run offline.
I think you have been fooled by EA's marketing.
The slower the game, the less often they have these hits to their servers.
[1] http://www.businessinsider.com/chris-kluwes-simcity-5-review...
I also ran into what many others have seen in the first few days. I built up a large city over a few hours and then quit. When I started back up, nothing was there. The city was claimed but it was a clean slate. You might see this from any number of things. Maybe the sim is all on the computer. Maybe the saves are just borked. Maybe the saves are fine on one node but haven't been replicated to whatever node I my request got load balanced to when i reconnected.