generally, passing by reference should be faster.
when you pass a struct by reference, you are only passing a pointer to the struct, which is only a 32/64 bit integer.
when you pass a struct by value, you need to copy the entire struct and then pass a pointer to the new copy.
unless the struct is very small, for example, an int, passing by reference is faster.
also, passing by value would increase the number of calls to the os for memory allocation and de-allocation, these calls are time-consuming as the os has to check a registry for available space.