How to increase computational speed by using SPMD command in Parallel Computing tool box in MATLAB!!!!

Hello,

I am learning parallel computing tool box in MATLAB. My CPU is Intel Core 2 Duo T 6400 2.00 Ghz and MATLAB R2009a. I executed two program one is sequential and another is parallel using SPMD command but i found that sequential program is taking lesser time i.e. 0.326433 seconds than parallel program i.e.3.985434 seconds. So please help me how to parallelize our sequential program using SPMD command.

Sequential Program is --

tic R = imread('cameraman.tif');

       [a b] = size(R);    

       for i = 1:a

           for j = 1:b

               if R(i,j) > 150

                   R(i,j) = 0;

               end

           end

       end  

          M = imread('cameraman.tif');

        [a b] = size(M);

       for i = 1:a

           for j = 1:b

               if M(i,j) < 150

                   M(i,j) = 255;

               end
           end
       end

imshow (R)

figure imshow(M)

toc

Parallel Program using SPMD is---

matlabpool open local 2

tic

spmd (2)

    if labindex==1 
       R = imread('cameraman.tif');
       [a b] = size(R);       
       for i = 1:a
           for j = 1:b
               if R(i,j) > 150
                   R(i,j) = 0;
               end
           end
       end

    else
        R = imread('cameraman.tif');
        [a b] = size(R);
       for i = 1:a
           for j = 1:b
               if R(i,j) < 150
                   R(i,j) = 255;
               end
           end
       end     
    end
end
imshow (R{1})
figure
imshow (R{2})
toc
matlabpool close

So please help me..I am very confused..