Performance Optimization is hard. Unity provides a lot of tools but not all are well known. Learn how to use the free tool, Hierarchical LOD (HLOD) to dramatically reduce draw calls and
Unity UI: My Top 3 Optimization Strategies | TheGamedev.Guru
Optimizing performance of Unity games - M2H
Unity 2017 Game Optimization: Optimize all aspects of Unity performance, 2nd Edition: Dickinson, Chris: 9781788392365: Amazon.com: Books