Not sure what your definition or filters are for low popularity but c# and .net in general purpose and application development are very popular according to various developer surveys. Depending on the survey often in the top five to ten.
I wouldn't say it's poor performing, definitely not rust, c or c++ capable, but you can write performant code through various tools, processes and optimisations, or bind to libs written in other languages e.g. rust, c++ to have them handle with that must be incredibly performant. Unity is a good example .net C# and C++ working together to deliver a performant solution.
Let me reiterate that I meant that it was previously low performing. I've developed in .Net since 2009 but only last few years have brought significant improvements in performance.
