Unity's Mono runtime executes C# code significantly slower than modern .NET, with benchmarks showing 2-3x slower performance in production and up to 15x in specific cases. The performance gap stems from outdated JIT compilation and poor runtime optimizations in Mono compared to .NET's CoreCLR. While Unity announced CoreCLR support in 2018, it remains unavailable in production as of 2025. Migrating to CoreCLR would deliver substantial performance improvements, faster iteration times, and access to modern C# features like Span<T> and hardware intrinsics. Assembly-level analysis reveals Mono generates inefficient code with excessive memory operations, while .NET produces optimized register-based instructions.