I'm assuming this is caused by the hardware you are using.
You could average out the pixel colors based on neighboring pixels. Iterate through each column/row and adjust your color based on the surrounding colors. Do several passes on the entire image to give a greater effect.
This wouldn't get rid of the noise completely but it would give a softer image.
Additionally, if you are passing this through a graphics card you might consider writing a Shader. This would dramatically increase the speed.