Parsing MWNumericArray to C# code optimization

This is my function returning matrices from Matlab. This code run very slowly. I was trying to use Parallel.For in-order to run it in parallel threads but I got different results than running it on one thread.

Am I iterating the matrix in a wrong order? Row, col vs col, row? Can you please review this code any suggest some performance boost tips?

public static SerializableMatrix ParseNumericMatrix(MWNumericArray matrix)
{
    SerializableMatrix resultMatrix;
    int[] dimensions = matrix.Dimensions;

    if (dimensions.Length > 3 || dimensions.Length < 2)
    {
        throw new ArgumentException("Input matrix dimensions are too long", "Three is the max dimensions");
    }
    var height = dimensions[0];
    var width = dimensions[1];
    resultMatrix = new SerializableMatrix() { Width = width, Height = height, Data = new List<double[]>() };

    if (dimensions.Length == 2)
    {
        var dataArray = new double[width * height];
        for (int i = 0; i < height; i++)
        {
            for (int j = 0; j < width; j++)
            {
                dataArray[i * width + j] = (double)matrix[i, j];
            }
        }
        resultMatrix.Data.Add(dataArray);
        return resultMatrix;
    }
    if (dimensions.Length == 3)
    {
        int depth = dimensions[2];
        for (int depthIndex = 0; depthIndex < depth; depthIndex++)
        {
            var dataArray = new double[width * height];
            for (int i = 0; i < height; i++)
            {
                for (int j = 0; j < width; j++)
                {
                    // matlab indices start at 1 , not 0!!!
                    dataArray[i * width + j] = (double)matrix[i + 1, j + 1, depthIndex + 1];
                }
            }
            resultMatrix.Data.Add(dataArray);
        }
    }
    return resultMatrix;
}