What Really Drives Unity Physics Cost? FixedUpdate, Collision Events, and Collider Complexity Explained
In Unity projects, physics performance is often underestimated until CPU spikes occur in production. This session breaks down the four core factors that determine physics cost: simulation mode, FixedUpdate frequency, collision event count, and collider complexity. Based on real device testing, we explain why physics can scale non-linearly, how FixedUpdate can create feedback loops under frame drops, and why different collider types follow fundamentally different computation paths even with identical shapes.
About GameOptim GameOptim helps Unity developers identify memory issues, rendering bottlenecks, and performance regressions through automated profiling and cloud based performance analysis. Explore more: 🌐 Website: www.gameoptim.com https://www.gameoptim.com/?fopt=blog 📘 Blog: www.gameoptim.com/blog/ https://www.gameoptim.com/blog/ 💼 LinkedIn: www.linkedin.com/company/gameoptim/ https://www.linkedin.com/company/gameoptim/ 🎥 YouTube: GO.PerformanceLab https://www.youtube.com/@GO.PerformanceLab 💬 Discord: GameOptim https://discord.gg/4Jh6hj9gRw ⭐ GitHub: GameOptim https://github.com/GameOptim/unity mobile performance guide 💻 Dev: GameOptim https://dev.to/gameoptim Physics simulation is one of the most performance sensitive systems in Unity, especially on mobile platforms where CPU budgets are tight and frame stability is critical. In this video, we revisit the core factors that influence physics cost and extend them with real world profiling insights from production testing. We break physics performance into four key dimensions: • Simulation Mode Auto Simulation, FixedUpdate, Script driven control • Number of Physics Updates FixedUpdate frequency and catch up behavior • Collision Event Count Contacts / Overlap and layer interaction design • Collider Complexity Mesh vs Primitive, and underlying computation paths We explain how FixedUpdate can unintentionally create a feedback loop: frame drops increase physics steps, which further increases frame time. We also show why default settings in Time configuration can lead to up to 17 physics updates per frame, a major hidden cost in mobile scenarios. Through layer collision matrix analysis, we highlight how improper layer design often causes unnecessary collision events, and how simple configuration changes can significantly reduce physics overhead. Finally, we present a controlled experiment comparing Sphere Colliders and Mesh Colliders with identical geometry. Results from a low end Android device demonstrate that even when visual complexity is equivalent, Mesh Colliders are consistently more expensive due to triangle based computation paths, while primitive colliders rely on optimized analytical calculations. Key takeaway: physics optimization is not only about reducing visible complexity, but about understanding the underlying execution model of the engine.