Speed of semi-transparency

The game i am making at the moment runs in 640x480, and uses a couple of active objects that are 640x480 with semi-transparency over the screen to fade it throughout the game. Does anyone know if this will slow down older PCs (it's made in MMF2). I also intend to include various transparent explosion effects.

Also, when it rains in my game, about 200 more small active objects are created, as well as a few semi-transparent clouds. Does anyone know if this would cause issues?

I'm just wondering whether i should include a "turn off weather and night-time" option.

On my computer it runs at a constant 50 frames a second, with both the transparent overlays and rain it drops to about 45 sometimes. My computer has 1gb ram and a 3.2ghz processor.

Any feedback from people who have experienced slowdown with mmf2 would be greatly appreciated.

I don't intend to upload an example of the game, as i'm keeping it secret-ish, even though some of you have played the demo.

200 extra little active objects won't be a problem. Those large semi-transparencies will be though on old PCs. So yes, include an option to turn off nighttime effects.

Ok, thanks a lot!