This was before MS was on board with Linux and all, but Mono was already super good tech. It wasn’t as fast as real .NET but it was still way faster than eg Python or Ruby, the popular backend languages of the day. There was even a half decent cross platform IDE (MonoDevelop), most of the dev team wasn’t on Windows.
I guess this is not very actionable knowledge anymore but I assume that if C# on Linux was productive and fast in 2013 it won’t have gotten any worse today.
Somehow linux/mac people seem to avoid .NET out of sheer cultural avoidance, and Windows people seem to avoid most of the OSS ecosystem like the plague for awkward “i was at a .net conf and all the talks were about MS tech” reasons but actually the two combine really well. C# is a great language with unparalleled tooling. It’s easy to learn for people with nearly any background. It’s fast, it’s easy to refactor, it’s easy to debug and it runs everywhere. For over a decade now.
My current company runs Elixir because we’re a chat API and the BEAM’s parallelism/robustness story is uniquely suited to our product. If we’d be in a different domain I’d choose C# (on linux, with postgres) again in a heartbeat.