How to increase the rendering speed?

Dear all:

I am trying to render an interior scene in daytime and nighttime. I would like to render many images with different material colors and textures in this scene, that means the geometry and illumination remain the same, but the materials on the wall, ceiling ,floor and so on change. The image size I need is very small(175*255 pixels). I need to render the images as faster as possible. So I wonder is there any way to increase the rendering speed?

I am using the Desktop Radiance2.0, I found it renders my model much faster than the Radiance 3.6 in cygwin. (2 min vs. 12 min. on same computer), and the results are different, the floor rendered by Radiance 3.6 in cygwin seems too much reflective. Are there any differences in the default settings of them? or is there something wrong with my installation? (Actually I just copy the model to cygwin directory and inpute the command: $ ./day.bat )

You can find my model here:

I think it may be not easy to unswer these questions, but your reply will be great help for me.