On runtime level there is no difference between structs and classes in C++ at all.
So it doesn’t make any performance difference whether you use struct A or class A in your code.
Other thing, is using some features — like, constructors, destructors and virtual functions, — could have some performance penalties (but if you use them you probably need them anyway). But you can with equal success use them both inside your class or struct.
In this document you can read about other performance-related subtleties of C++.