Interesting. I tested this using the same circumstances Graslu did and my FPS only hovered into the 100s when i detonated 10 mines right in front of myself with all the fancy effects on. I actually only get 600 or so in that corridor when nothing's happening, so it's not nearly as colossal a drop.
However, there is certainly something going on here. When i placed the same 10 mines in the control room and detonated them while standing in the cells, my FPS rapidly fell to 100 before climbing back up again at roughly the same rate. I normally cap at 120, which explains why I've never noticed it before, but I've tested it on a few maps and the same thing seems to happen everywhere.
I tested this with everything at max, but without HDR and motion blur. I use an AMD Phenom 2 X4 965 processor with a Radeon HD 6850 card, so there's a bit of a difference there, but i can't say I've heard of anyone who runs Intel complaining of this problem.
Regardless, here's hoping we can narrow down the potential causes a bit, because this issue could be more widespread than first suspected.