Simplifying a little (well... a lot), drawing the screen involves sending the geometry of everything in the scene to the graphics card, then telling it how to colour in the bits.
Each of these instructions to draw something using a particular material is a "draw call". Now, the graphics card is a very, very fast painter, but each draw call (changing paint cans) involves a noticeable amount of work. In short, it's much faster to paint many walls with the same paint (material) than it is to change cans for every wall.
In the beginning, we were painting every object on the screen with its own can of paint. That's not just each building - each building consists of at least four legs and a body. Even though all the legs of a building have the same colour, they still got their own can of paint, so we were quickly up to thousands of draw calls, making the game run quite slowly unless you had a fast CPU. In the meantime, the graphics card was sitting there, waiting for the CPU to hand it more paint cans.
The first step we took was to redo our models and make sure all the matching bits (like the legs) actually share a single material. Then, Unity can use dynamic batching to draw all the blue bits with the same paint can (one single draw call). Then, all the red bits get drawn in one go, etc. Big win!
Unfortunately, with dozens of buildings on the play field, all consisting of several pieces, rendering shadows for all those little bits meant it was still too slow. We couldn't just rely on Unity combining the draw calls for us. We needed to draw less things! We don't actually have to leave anything out, because graphics cards are so fast these days. We just need to draw bigger chunks at a time.
So there you have it. Powargrid should run a whole lot smoother, with nice shadows, even if you don't have a very fast PC. Enjoy!