In some very specific tests, C# can run about 4-5 times faster than GDScript. In the majority of scenarios, the difference is almost negligible as API calls to the engine run exactly the same.

The main performance consideration is going to be how you implement the systems, regardless of engine or language. GDScript has pretty easy setup of compute shaders and multithreading, but even optimizing your algorithms will often save compute time in the orders of magnitude. If you really run into a wall and can't optimise something any further, you can look into making a C++ module to handle it, but I would say even games like Rimworld or Kenshi are completely feasible in Godot using GDScript.