I just checked, and my iMac doesn't even have an integrated graphics card. Only the Geforce GTX 680M ... So steam MUST be using this card, as there are no other choices? This is FUBAR!
I can understand being frustrated, but there are certain realities you must come to terms with.
1) Apple as a developer and Mac/OSX as a computing platform, have a history of being just awful when it comes to games compatibility. There are plenty of places on the net to check if a game will run well on a specific model of Mac computer. If you insist on gaming with a Mac computer, expect issues. Linux people have the same types of problems (but for different reasons).
2) The GeForce GTX 680M, while being a very capable mobile processor, is still a mobile graphics processor. There are many concessions made to the chip die to keep the overall thermal profile within mobile computing platform limits. There have been very limited reports of those who externally cool the mobile GPU to try and fool the OS into not throttling the performance of the GPU under heavy load.
3) The Apple furnished GPU drivers, again historically, have had relatively lackluster OpenGL performance. When I had my MacBook Pro and was trying to get a specific game to work, I had to downgrade the graphics driver to make sure the driver had the proper OpenGL features for a specific game. But that broke my World of Warcraft install. So I had to build scripts to swap between the 2 versions of graphics drivers and having to reboot the computer to use the different drivers.
4) Cities: Skylines uses Unity as the graphics/physics gaming engine for the game. This is very good news for game publisher and the consumers, since the developer gets to support all the computing platforms the engine supports and the leading edge of quality games for all platforms is moved forward a tiny amount. This is a benefit for the entire gaming industry.
It also means that native graphics support by Paradox is really out of their direct control. They are reliant on how well the Unity engine supports a particular platform. The unfortunate and ugly reality is there will always be edge cases where a particular computer model or even a specific computer has just the wrong components so it won't work well. That is what is most likely happening in your case. Your frustration, while perfectly understandable, is misplaced. You should be looking and researching the Unity forums to see if there are fixes you can implement on your end to allow the Unity engine to work optimally for your computer.
5) Finally, your insistence that booting to the BootCamp partition just to play C:S is not an option, you'll have to take that up with Apple. Apple has always had the opinion that their computers are not gaming machines and if you really want to game on their computers, use the free BootCamp dual boot ability. Hopefully, Tim Cook won't be as hostile to gaming as Steve Jobs was. We'll all have to wait and see on that one.
