I have the same problem, i've got a bunch of different plasma effects, but to be of any use, they need to cover at least the whole screen, there's three ways i can think of to lower the lag but haven't done, one of which doesn't exist
1. Make it so whenever there is an effect like an explosion or something, create a plamsa object around the vicinity where it will be needed. This does cause some problems for scolling screens/objects though
2. Make it take up the whole screen/level, but make it invisible when it isn't in use. Good in theory, hard in practice since if there are a lot of effects on screen, it will still lag.
3. Bug the devs to make it so when nothing is happening on the plasma object, it doesn't render what whatever it does that makes it lag
That's all i got