Not only code optimization, but also memory management, scene management, culling, streaming, organizing a database of tens of thousands of game entities and fastly accessing to it via optimized data trees and so on.

Actually a renderer is just a renderer. The pure sense of an engine is to do what I wrote and much more.

Because of that I was not perfectly happy with Unity when I compare it to Vision3d or UDK. The optimization and culling was not that impressive. But they are addressing it and already announced a lot of these optimizations in the next few versions. That is the right way to go.