We (Mr Matti and me) just tested it with DirectX 9 and 800x600 pixels and it worked fine on a 2x2 GHz with a bad video card (sis mirage 3)! There is no slowdown.
We used 6 objects with the PxPos -3 to 2 with the factors 1, 0.9375, 0.875, 0.75 and 0.5 in order to have each object scrolling with half the speed of the previous one which creates the effect seen in the picture above.
With every object exept one it works fine, we didn't succeed to get it do what we want and now we are close to getting mad. It seems to be not affected by the lines of code that we think are the right ones...
Honestly we have problems to fully understand that crazy move-with-scrolling-speed-in-opposite-direction-than-scrolling-math-thing.
We uploaded the game folder
here. It would be nice if you could take a look at the script and tell us what obvious mistake we made.