Earlier quoted context omitted.
Most Windows apps that are guilty of being slow and poorly made do not use the .NET you speak of at all. If they did they would work much better!
Do you have some examples of large and complex C# applications with really good performance? I’m hard pressed to think of any Microsoft software at all that I think feels really nice. Except for VSCode, which is of course built on Electron and not .NET. The only thing that comes to mind is Paint.NET, which doesn’t seem like all that big of an application. It’s very nicely done, but I think it’s only got like a single…
Most of them. An implementation would have to go out of its way to make itself perform poorly if it is built on top of, say, .NET 8.
Have you heard of Ryujinx? Or Nethermind? Or Garnet? Or have you seen that .NET's Regex engine, written in pure C#, is the third fastest in the world after Intel's Hyperscan and Rust's Regex crate?