Post #80: A Note About Godot & C# Marshalling
Godot’s FAQ section details a great note about the speed of GDScript versus the other Godot supported languages (C# and C++, though there are more via GDExtensions, of course).
There is a key note worth sharing; C# might sometimes be faster than GDScript, but it can be less performant due to marshalling. You can check out the Wikipedia article for the definition, but what you basically need to know is that transforming lots of data for use by the C# runtime and passing it back and forth to Godot could be less performant than just using GDScript.
Unless you are planning on using C++ or another systems language, it may be worth seeing if pulling in C# is really worth the added complexity, especially if GDScript can do the job just fine!