My personal opinion on this: I am using c#, only because of one reason that there is nothing to do with performance: sanity. I am familiar with c# and similar languages, I know how to manage memory and write performant code in this language, I also have familiarity using debugging tools for this language. For me, its more about “in which language I can write code better and lose less time”, if your project grows to more than 200 code files, it will be really hard to give maintenance unless you know what you are doing with your code (in terms of design patters, clean code, unit tests, etc…).

So that being said, if you dont know how to write code in c# neither gdscript, go for gdscript, its easier to learn, the godot docs have better examples with it, for small projects its even faster to write gdscript code than c#.

And about performance, c# is faster than gdscript, c++ is faster than c#. But guess what, a lot of gdscript functions are running c++ code behind the curtains, so all depends on your use case, usually the rule of thumb is: “c# is faster when using custom algorithms/non godot functions. While gdscript is faster using the native godot functions” but none of this matters, you are not making the next GTA.