If you are seeking for performance above all, C++ is the way to go.
GDScript is ~100 times slower than C++, so you should only use it for very high level stuff, like "do this when walking in this area", or "Spawn enemy here every X seconds".
The best tradeoff is to find what requires the most CPU, implement it in C++ and expose high level functions to GDScript/Nodes/Resources, so you can achieve great level of tweak with good performance.