For a long time many of the .NET Runtime's most fundamental and performance-sensitive functions were written in C++. In the beginning the JIT wasn't fast enough

For a long time many of the .NET Runtime's most fundamental and performance-sensitive functions were written in C++. In the beginning the JIT wasn't fast enough, or C# didn't have good enough native interoperability, or it didn't have the tools to write fast code. As the years went on the JIT got faster, and more and more improvements were added to help write high performance C# code. Now there's a mass effort to port most of the runtime to C# to make it safer, more maintainable, and often faster. RyuJIT may not generate code as nice as LLVM but there's so much overhead calling into native code that there are gains to be had. Imagine the alternate timeline where numpy is removing some of its C function calls because Python is fast enough on its own. I'm not sure we can make it there from here.