I have a class which is frequently copied in a performance critical section of code. The class has a string member variable which may be set multiple times during the an object's lifetime but ultimately isn't used for anything meaningful. For obvious reasons I would like to remove this variable.

class MyClass {
public:
  //Constructor
  ...
  //Methods
  ...
  //Properties
  double usefulMember;
  int usefulMember2;
  std::string uselessString;
};

However, I have found that if I remove uselessString then my code runs ~15% slower. Can somebody explain why this would be? If anything I would think that getting rid of the need to copy this string over and over would improve performance.

One thing I noticed is that with the uselessString variable sizeof(MyClass) is 448 (divisible by 64). Could this have to do with how MyClass aligns with the CPU cache? I tried declaring the class with alignas(64) but it didn't help.

Edit: I ended up using the Intel TBB malloc_proxy to implement scalable memory allocation across my whole program. I don't know exactly what the issue was before but things are now running faster than ever before.