C# has faster execution speed compared to GDScript, but Godot needs to marshal data when communicating between the core engine and the C# runtime, which can negatively affect performance if you don't keep it in mind when writing your code.

But ultimately I'd argue compilation speed is waaay more important than execution speed when it comes to creating games (which require rapid iteration and testing). There's only a small amount of code in a codebase where squeezing out the maximum performance is required, and those important bottlenecks can be refactored or rewritten in a more performant language if needed.