Take them out.
Seriously, complex particle effects (and lots of them happening at once) will kill your framerate. You can do a number of things to help improve the framerate, but if you want to get it back up to full force, ditch them.
1) - Reduce the emmission volume. If the effects are something like rain, or smoke try reducing the total number of particles that are put out.
2) - Use a smaller particle. It takes time to draw each particle on the screen, and if each one is a large image, it takes even longer. Depending on what you are currently using you should be able to see an improvement by reducing the image size.
3) - Kill them. Make sure that you don't have the particles hanging around for too long. Again this ties in with the first point, but it is a little bit different. If you have a smoke effect where each particle fades out slowly over 10 seconds or so, you will have a whole crap load of particles even at just one per frame. Now if you combine that with a bunch of smoke effects you are now sitting on 10 crap loads of particles that the player will not even notice...so you might as well kill them off quicker.
