Why is std::atomic<bool> much slower than volatile bool?

I've been using volatile bool for years for thread execution control and it worked fine

// in my class declaration
volatile bool stop_;

-----------------

// In the thread function
while (!stop_)
{
     do_things();
}

Now, since C++11 added support for atomic operations, I decided to try that instead

// in my class declaration
std::atomic<bool> stop_;

-----------------

// In the thread function
while (!stop_)
{
     do_things();
}

But it's several orders of magnitude slower than the volatile bool!

Simple test case I've written takes about 1 second to complete with volatile bool approach. With std::atomic<bool> however I've been waiting for about 10 minutes and gave up!

I tried to use memory_order_relaxed flag with load and store to the same effect.

My platform:

• Windows 7 64-bit
• MinGW gcc 4.6.x

What I'm doing wrong?

NB: I know that volatile does not make a variable thread-safe. My question is not about volatile, it's about why atomic is ridiculously slow.