From 60 to 15 fps w/ fire nova mines

Hi,

I've a quite old conf (C2Q Q9400 @2.67Ghz, ATI 4870 1Go, 8Go RAM) and i think i'm gpu-bound*. The multithreading helped a little as i think i've quite less fps spikes, but with the build i'm currently playing, each time i detonate more than 3 mines, i've a massive fps drop.
I put some screenshots on Imgur (before, while and after detonation).

I'd like to know if there's an option to see (like in the video regarding the multithreading with the different colored bars) what's causing this. I guess it's a dev tool only but who knows.

Besides, is there some additional video options that i could edit to have less impact on my gpu when there's too many effects displayed ?

*I mean i was already, but with the 2.4, the CPU probably isn't a problem anymore.
Another feedback related from another player.