problem with comparing images in parallel

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C#

C#4.0

I have build an application in c# with .NET 4.0 to compare two images using parallel library. I have a PC with two cores, but when i run the app in one core i runs faster than when i run it in two.

this is the method that compares images.

private unsafe Bitmap krahas(Bitmap nje, Bitmap dy)
        {
            Bitmap a = new Bitmap(nje);
            Bitmap b = new Bitmap(dy);
            int gjersia = Math.Min(a.Width, b.Width);
            int lartesia = Math.Min(a.Height, b.Height);
            Bitmap rez = new Bitmap(gjersia, lartesia);
            int numri_i_procesorve = Numri_i_berthamave.Value;
            //Bëjmë lock bitat e fotove
            Rectangle per_a = new Rectangle(0, 0, a.Width, a.Height);
            Rectangle per_b = new Rectangle(0, 0, b.Width, b.Height);
            Rectangle per_rez = new Rectangle(0, 0, gjersia, lartesia);
            BitmapData t_per_a = a.LockBits(per_a, ImageLockMode.ReadWrite, a.PixelFormat);
            BitmapData t_per_b = b.LockBits(per_b, ImageLockMode.ReadWrite, b.PixelFormat);
            BitmapData t_per_rez = rez.LockBits(per_rez, ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);

            byte* adresa_per_a = (byte*)t_per_a.Scan0;
            byte* adresa_per_b = (byte*)t_per_b.Scan0;
            byte* rezult = (byte*)t_per_rez.Scan0;

            Color cl = Color.Red;
            byte[] ngj = new byte[4];
            ngj[3] = cl.A;
            ngj[2] = cl.R;
            ngj[1] = cl.G;
            ngj[0] = cl.B;

            int rowpadding = t_per_a.Stride - (a.Width * 4);
            ParallelOptions po = new ParallelOptions();
            po.MaxDegreeOfParallelism = numri_i_procesorve;
            koha_e_kaluar.Reset();
            koha_e_kaluar.Start();
            object lok = new object();
            Parallel.For(0, lartesia, po, i =>
        {
            lock (lok)
            {
                for (int j = 0; j < gjersia; j++)
                {
                    int same = 0;
                    byte[] tmp = new byte[4];
                    for (int x = 0; x < 4; x++)
                    {
                        tmp[x] = adresa_per_b[0];
                        if (adresa_per_a[0] == adresa_per_b[0])
                        {
                            same++;
                        }
                        adresa_per_a++;
                        adresa_per_b++;
                    }
                    for (int x = 0; x < 4; x++)
                    {
                        rezult[0] = (same != 4) ? ngj[x] : tmp[x];
                        rezult++;
                    }
                }
                if (rowpadding > 0)
                {
                    adresa_per_a += rowpadding;
                    adresa_per_b += rowpadding;
                    rezult += rowpadding;
                }
            }
        });
            koha_e_kaluar.Stop();
            a.UnlockBits(t_per_a);
            b.UnlockBits(t_per_b);
            rez.UnlockBits(t_per_rez);
            return rez;
        }

I don't now where is the problem. Can anyone help me! Please.