Best and fast :)

You change initial condition

• from: std::string DNAStrand(const std::string& dna)
• to: std::string DNAStrand(std::string dna)

Thus you made two copies of string, first - when signature passed, second - when return value

#include <iostream>

class AAA 
{
public:
    AAA ( ) {}
    
    AAA ( const AAA& other )
    {
        std::cout << "copy" << std::endl;
    }
} ;

AAA func ( AAA a )
{
    return a;
}

int main()
{
    AAA _a;
    AAA _b = func(_a);
}

Output is:
copy
copy

Therefore it is a BAD practices ...