Running time in Heap - Stack

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C#

I created a code in order to calculate the time of running.
It's working. But I couldn't explain: What is the difference between variables in Heap and Stack?

I'm talking about "running time".
I created 3 variables in Heap: a, b, c.
And 3 variables in Stack: aa, bb, cc.

My code:

class Program
{
    private int a = 1;
    private int b = 2;
    private int c = 0;

    static void Main()
    {
        int aa = 1;
        int bb = 2;
        int cc = 0;

        var sw = new Stopwatch();
        var _sw = new Stopwatch();

        Program pro = new Program();

        for (int k = 0; k < 10; k++)
        {
            sw.Start();
            for (int i = 0; i < 500000000; i++)
            {
                pro.c += pro.a + pro.b;
            }
            sw.Stop();

            Console.WriteLine("Heap:");
            Console.WriteLine("TotalMiliseconds: {0}", sw.Elapsed.TotalMilliseconds.ToString());
            Console.WriteLine("__________________________________");
            sw.Reset();

            _sw.Start();
            for (int j = 0; j < 500000000; j++)
            {
                cc += aa + bb;
            }
            _sw.Stop();

            Console.WriteLine("Stack:");
            Console.WriteLine("TotalMiliseconds: {0}", _sw.Elapsed.TotalMilliseconds.ToString());
            Console.WriteLine("__________________________________");

            _sw.Reset();
        }
        Console.ReadKey();
    }

Here is my question: Can you tell me why the running time in Heap is always faster than Stack?
Thanks!