For what I hack on I often run into issues with the FFI performance between go/c# <-> c/c++. I’d rather not write C or C++ and Rust is one of the few languages that allows me to mess around with obscure libs at native performance and yet slap a web interface in front of it. cxx/bindgen is stellar in how fast they allow you to wrap libraries. These cases is where I would want like the simplest most opinionated web framework (like gin in Go).
Have you had real success with cxx? I've been trying to FFI a C++ lib (that I have limited control over) and it's been a real PITA. At this point I'm thinking of just making a pure-c interface for what I want on the C++ side and using C bindgen or just straight "extern C" because C++ FFI seems so painful.
FFI can be a source of performance issues in Go but not in C# (at least not to the same extent), unless you go out of your way to fight the happy path approach.
Sometimes you do have to rethink what you marshal vs what you just manage manually, but that’s what pointers are for (that can be turned into ref Ts for single values and into Span<T>s for slices/arrays/etc). The idea is that performance ceiling of FFI in .NET is as cheap as direct calls.
For everything else, there’s Go IMO.