Unity's Mono Problem: Why Your C# Code Runs Slower Than It Should

Execution of C# code in Unity’s Mono runtime is slow by today’s standards, much slower than you might expect! Our game runs 2-3x faster on modern .NET compared to Unity’s Mono, and in a few small benchmarks I measured speedups of up to 15x. I’ve spent some time investigating what’s going on and in this article I will present my findings and why everyone should want Unity’s .NET modernization to become production-ready as soon as possible.

Unity uses the Mono framework to run C# programs and back in 2006 it was one of the only viable multi-platform implementations of .NET. Mono is also open-source, allowing Unity to do some tweaks to better suit game development.

An interesting twist happened nearly 10 years later. In 2014, Microsoft began open-sourcing .NET (notably .NET Core later that year) and in June 2016, .NET Core 1.0 shipped with official cross-platform support. Since then, the .NET ecosystem gained momentum and lots of improvements have been made, including the Roslyn compiler platform, a new JIT (just-in-time compiler), performance improvements, more features, etc.

In 2018, Unity engineers discussed that they are working on porting the engine to .NET CoreCLR, the multi-platform version of Common Language Runtime (CLR), a component that runs .NET programs. Their main motivations behind this project were performance and convergence. In their post they said:

Unfortunately, now it’s the end of 2025 and we still can’t run games on CoreCLR.

We don’t hear about the performance gap between Mono and .NET much, likely because it is not possible to run games written for Unity under modern .NET. But we can still do a direct comparison with code that does not depend on Unity directly.

Our game has a unique architecture – we strictly separate the game simulation code (business logic) from rendering. So much so that the simulation code does not depend on Unity’s libraries and can be compiled and run under any .NET version.

One day I was debugging an issue in map generation and it was time-consuming because it was taking over 2 minutes to start a game. To make debugging faster, I’ve written a unit test, hoping to cut down on the turn-around time since Unity takes 15+ seconds just to crunch new DLLs and reload the domain before the game can be launched and it also initializes rendering stuff that I did not care about. When I ran the test, it finished in 40 seconds. I was quite surprised that it was more than 3x faster, so I started digging deeper.

Source: HackerNews