Campeador I found something to help prevent slowdown when I have lots of fighters (about 20 or more fighters) firing their weps. I took the effects bitmap of a torpedo bitmap and reduced it by 50% to 128 x 128 with no real noticable game difference in quality. By doing this my wep effects were firing much better. There was no slowdown as there was previously when I had the effects file at 256 x 256. It was as if the fighters were on "steroids" or something.
Another thing I try to do is keep the texture images sizes down to 256 x 256 for fighters and most ships for the most part. I do have some higher 512 x 512 for ships and what not. I noticed a lot of the fighters in multiverse have texture size 512 x 512. Try it out and reduce it down to 256 x 256 and see what you think. It will save memory use in the game.
Something else I want to share with you that I stumbled upon is that you can use jpg instead of bitmap for landscapes and portrait images. Just reference the images in the files as jpg instead of bmp. Big-time savings on file sizes. You may not what to mess with this now since it would take too much time to convert but in the future it is good to know.