Passing a string by value, reference and rvalue

I'm just comparing the performance of passing a string to a function. The benchmark results are interesting.

Here's my code:

void add(std::string msg)
{
    msg += "world";
}

void addRvalue(std::string&& msg)
{
    msg += "world";
}

void addRef(std::string& msg)
{
    msg += "world";
}

void StringCreation() {
    add(std::string("hello "));
}

void StringCopy() {
    std::string msg("hello ");
    add(msg);
}

static void StringMove() {
    std::string msg("hello ");
    add(std::move(msg));
}

void StringRvalue() {
    std::string msg("hello ");
    addRvalue(std::move(msg));
}

void StringReference() {
    std::string msg("hello ");
    addRef(msg);
}

StringCreation(), StringRvalue() and StringReference() are equivalent. I'm surprised StringMove() is the least performant - worse than pass by value which involves a copy.

Am I right in thinking that calling StringMove() involves one move constructor followed by a copy constructor when it calls add()? It doesn't just involve one move constructor? I thought move construction was cheap for a string.