Program.cs

using System.Diagnostics;

float[,] image = new float[10000, 10000];
var rng = new Random();
for (int i = 0; i < image.GetLength(0); i += 1)
{
    for (int j = 0; j < image.GetLength(1); j += 1)
    {
        image[i, j] = ((float)rng.Next()) / Int32.MaxValue;
    }
}

float[,] accum = new float[image.GetLength(0) - 2, image.GetLength(1) - 2];


Stopwatch timer = new Stopwatch();
timer.Start();

// for (int i = 0; i < accum.GetLength(0); i += 1)
// {
//     for (int j = 0; j < accum.GetLength(1); j += 1)
//     {
//         accum[i, j] = 1;
//         for (int k1 = 0; k1 < 3; k1 += 1)
//         {
//             for (int k2 = 0; k2 < 3; k2 += 1)
//             {
//                 accum[i, j] += (float)Math.Exp(image[i + k1, j + k2]);
//             }
//         }
//     }
// }

ParallelOptions options = new ParallelOptions();
options.MaxDegreeOfParallelism = 2;
Parallel.For(0, accum.GetLength(0), options, (i, state) =>
{
    for (int j = 0; j < accum.GetLength(1); j += 1)
    {
        accum[i, j] = 1;
        for (int k1 = 0; k1 < 3; k1 += 1)
        {
            for (int k2 = 0; k2 < 3; k2 += 1)
            {
                accum[i, j] += (float)Math.Exp(image[i + k1, j + k2]);
            }
        }
    }
});

timer.Stop();
Console.WriteLine(timer.Elapsed);