C++11 vector

I notice that vector is much slower than bool array when running the following code.

int main() 
{
    int count = 0;
    int n = 1500000;
    // slower with c++ vector<bool>
    /*vector<bool> isPrime;
    isPrime.reserve(n-1);
    isPrime.assign(n-1, true);
    */
    // faster with bool array 
    bool* isPrime = new bool[n-1];

    for (int i = 0; i < n; ++i)
        isPrime[i] = true;


    for (int i = 2; i< n; ++i) {
        if (isPrime[i])
            count++;
        for (int j =2; i*j < n; ++j )
            isPrime[i*j] = false;
    }

    cout <<  count << endl;
    return 0;
}

Is there some way that I can do to make vector<bool> faster ? Btw, both std::vector::push_back and std::vector::emplace_back are even slower than std::vector::assign.