Problem is, GDScript still doesn't have at least a JIT, and the whole GDextensions boilerplate seems more complicated than it needs to be.
If performance is a concern, skip C# and go straight to c++. Now your ripping at max speed with the smallest binary! That's my whole point. GDScript + c++ is my point. Ditch C# it's not worth the squeeze.
Interpreter code is never faster than a dynamic compiler, otherwise what about doing games in Python?
As mentioned on my comment, GDextension experience kind of sucks.
I'm offering real practical advice from experience of having worked on real projects.
I'll make it real clear:
GDScript & c++ > C#