Oh, I see, you mean to use particle emission every few frames. I see why you'd think that, but ultimately that would seriously multiply the number of sprites that need to be drawn every frame in heavy-missile combat, and that would really chug things in a hurry. Drawing shots is one of the big performance hogs in AIW on the graphical side. There's a lot of sprite batching and such to avoid texture swaps but it just gets to be quite a lot of draws and so on.