Graphically speaking, Warp plasma and Theta vent are called particles effects, and they are probably the biggest resources hogs of any graphic. There is a option in the game menu to adjust the detail level of particles effects separately so the first thing I would try would be to change that to the next lower setting. If that doesn't change anything then it's more likely that your system overall is having problems with something that takes so many resources. What are the rest of the specs of your computer, and what type of internet and how good is your connection? (ie. ping, download, upload, packet loss)
I've used these types of buffs and haven't had any noticeable lag. I have all my graphics settings on maximum both in game and in the Catalyst control.
Add me to the list, I'm running a Phenom II X4 @ 3.6ghz, 8GB RAM, Geforce GTX480 and get about 7-12 FPS anytime thermal vent is used, Warp Plasma doesn't cause any problems for me.
@VoicesInTheDark: Internet ping and speed "shouldn't" have nothing to do with graphical effects.
You're right normally it shouldn't, however if you internet connection is unstable the lag from that can exaggerate the warp plasma/vent lag.
However with the system specs you have there is no reason why you should only be getting 7-12fps. if you have the extra hard drive space you might want to try increasing your virtual memory size. I've noticed sometimes windows doesn't always do the greatest job managing the virtual memory on some games. I upped min e to 10gigs for the virtual memory and it made a huge difference with particle effects on all the games I have.