std::async vs std::promise

I have to wrap a getter function into a std::future object. So simple question, what is the best / fastest way to do this?

Here are two options I came up with.

I have a function:

std::function<String (String)> getter;

Then wrap this using std::promise:

std::function<std::future<String> (String)> binding = [getter](String context) {
    std::promise<String> p;
    p.set_value(getter(contex));
    return p.get_future();
};

Or using std::async:

std::function<std::future<String> (String)> binding = [getter](String context) {
    return std::async(std::launch::deferred, getter, contex);
};