Rendering graphics in Unity is a complex task. For a basic understanding of how it works, see the Unity documentation on the Rendering Pipeline documentation before reading this guide. This guide provides a better understanding of the underlying concepts related to rendering, and best practices for reducing the rendering workload on the GPU.
To effectively optimize your graphics rendering, you need to understand the limitations of your target hardware, and how to profile the GPU. Profiling helps you check and verify that the optimizations you’re making are effective.
When profiling, it’s useful to start with a benchmark. A benchmark tells you what profiling results you should expect from specific GPUs when your lighting is running optimally.
See the GFXBench website for a great list of different industry-standard benchmarks for GPUs and graphics cards. The website provides a good overview of the current GPUs on the market, and how they compare against each other.