December 28, 2025
Mono? More like slo-mo
Unity's Mono problem: Why your C# code runs slower than it should
Fans say Unity’s stuck in 2006 while rivals speed past — “just use Stride” is the new mantra
TLDR: A dev shows Unity’s old Mono tech makes C# run much slower than modern .NET, and Unity’s promised upgrade still isn’t here. Comments explode into “switch engines,” IL2CPP skepticism, and garbage collector complaints, with the crowd split between ditching Unity now or waiting for a faster editor.
Unity dev Marek Fiser says C# in the Unity editor runs in Mono, an older engine tech, and it’s painfully slow. His tests show 2–3x faster on modern .NET (and up to 15x in tiny benchmarks). Unity promised a switch to Microsoft’s newer runtime back in 2018 with “2x–10x” gains (link), but it’s late 2025 and still not here—and the comments are on fire.
One camp is done waiting: “Unity’s lost the skillset,” says a fed‑up dev, waving people toward Stride (link). Another camp pushes back: a tester found Mono and CoreCLR “essentially the same” in single‑thread tests (link), hinting the slowdown might be about editor overhead, multiple threads, or how Unity glues pieces together. Then comes the reality check: pros ask if CoreCLR even matters when release builds use IL2CPP (Unity compiles C# to C++ for speed), which already runs way faster than the editor. And the plot twist? A commenter claims Unity’s garbage collector—the memory cleanup system—is the older Boehm GC and “performs very poorly” versus modern .NET.
The mood is spicy, with memes about the editor loading slower than a microwave dinner and one snarky jab: “Very obvious AI writing.” Devs are split between abandon ship and wait‑and‑see, but everyone agrees: faster playtests and fewer coffee breaks would be a game changer.
Key Points
- •The author measured their game running 2–3x faster on modern .NET compared to Unity’s Mono, with some small benchmarks up to 15x.
- •Unity adopted Mono around 2006 for multi-platform .NET, while .NET was open-sourced in 2014 and .NET Core 1.0 shipped in June 2016.
- •Unity engineers in 2018 discussed porting to CoreCLR, projecting 2x–5x performance gains and up to 10x on some workloads.
- •By the end of 2025, the article states that games still cannot run on CoreCLR within Unity.
- •A profiler-based benchmark showed 100 seconds for save load, map generation, and simulation init under Unity/Mono versus 38 seconds under .NET (Debug mode; Unity 6.0 vs .NET 10).