If you want full performance, you would do everything 100% in C++. Blueprints are useful when dealing with things that are difficult to put into code, like linear algebra, calculus, Physics, Animations, ect. Assuming your C++ knowledge is strong and you know how to optimize your code, C++ will be faster in almost all cases. There are some situations where blueprints are not practical, like an inventory system or anything dealing with recursion.