Raw code speed on iOS

I am testing raw performance in Xamarin. To do this I have a simple Prime number generator. Implemented in Obj-c Xcode it takes 9 seconds, but in C# on Xamarin it takes 78 seconds. If I run in the emulator it is 800ms v/s 1600ms. I am doing a release build, with Optimization etc. Is Xamarin just generally slower for raw code execution? My code is here...

// C# version
public static CalcResult Calc (int max)
{
    CalcResult res;

    int maxPrime = 0;
    Stopwatch sw = new Stopwatch();
    sw.Start();

    for (int i = 3; i < max; i += 2)
        {
        bool prime = true;
        for (int j = 2; j < (i / 2) + 1; j++)
        {
            if ((i % j) == 0)
            {
                prime = false;
                break;
            }
        }

        if (prime)
        {
            maxPrime = i;
        }
    }

    sw.Stop();
    res.elapsedMilliseconds = sw.ElapsedMilliseconds;
    res.largestPrime = maxPrime;
    return res;
}



// Obj-C code version
+(int)CalcPrime:(int) maxInt
{
    int maxPrime = 0;

    for (int i = 3; i < maxInt; i += 2)
    {
        bool prime = true;

        for (int j = 2; j < (i / 2) + 1; j++)
        {
            if ((i % j) == 0)
            {
                prime = false;
                break;
            }
        }

        if (prime)
        {
            maxPrime = i;
        }
    }

    return maxPrime;
}