C# dev here, mainly working in enterprise size companies who are Microsoft shops. We usually deploy on Windows for on-prem services, and on Linux for cloud services (some on VMs, some on containers, some serverless).

I'm more of a Windows guy for desktop, but always prefer Linux for server. Nowadays dotnet is a first class citizen on Linux and MacOS, through dotnet core.

On performance, there has been a big focus on perf by the dotnet team over the past years - I'd be shocked if Java could beat dotnet on just about any metric. If you know what you're doing you can write allocation-free code, and you even have hardware intrinsics at your disposal - I recently ported a hashing algorithm to C# and got the performance pretty close to parity with native C code (almost memory speed).

C# doesn't get much love on HN, but IMO it's a truely fantastic language.