For weaker cards, it is useful to experiment :
1. Turn off all forms of AA (I do that, and I still find the game very beautiful : I do not notice the jagged edges.)
2. Turn off "bloom".
3. Outside of the game, in your ATI Catalyst graphical-management utility, find the "V Synch" option and turn VS off (I have a GeForce, and we do it there, in its equivalent utility program.)
4. I turn off a few minor features such as the planet "elevators", the "dust", the asteroid field. (I don't care for the tiny, zooming little vehicles moving around the planet, and seeing a few more (very small) asteroids floating around does not enhance my visual experience.)
5. You have a series of buttons, in the options, which define the level of texture quality of ships, buildings, etc. I set them at the "highest" level, but certainly, weaker videocards should have lower settings for those buttons (experiment).
6. Of course, the more star-systems and planets, and the more players you put into a game, the more complex data will have to be managed by your videocard, your processor, and your RAM. (When I play alone against the A.I., I always create a game with only one star and not more than 30 planets -- also for a quicker game.)
7. Start by setting the game in the "native resolution" of your monitor, and if you still experience framerate slowness despite having experimented with tips 1 to 6, lower your resolution (for the game, not the desktop!).
8. The game has 3 speed settings : slow, normal, fast. I do not know what effect the fastest speed might have on weaker machines, but my guess is that the fast speed must provoke a slow framerate when the game has reached a point where many ships are doing battle -- much more data must then be computed (by the CPU and the GPU) at each second. Try lowering the speed.