Unfortunately it’s a fact that Cities: Skylines runs worse on other platforms than Windows. Now things might get a little better.
Some Linux users have been requesting Colossal Order to update Cities: Skylines to the latest Unity 5.4 engine, since it’s supposed to bring some performance improvements for Linux users. In case you didn’t know, Unity is a game engine that Cities: Skylines uses for rendering. Colossal Order’s response to those requests was that they will most likely be able to update the engine when the new expansion hits the market.