Funny story about Kuuterselka.
During testing we got some very strange FPS drops that only happened when there were more than 20 people on the server. We figured (logically) that it might be a problem with the new player models, so we had everyone meet up in the centre of the map, but oddly, there was now no lag at all.
As we're discussing what the problem might be, the beta testers start screwing around and suddenly we notice the lag creeping in again.
Getting an idea, I asked everyone to suicide and come back with either a PPsH-41 or a KP/-31 Suomi. So everyone meets up again and I say "Ok, at the count of 3 shoot your entire magazine up into the air. 3... 2... 1...".
My FPS dropped from 60 to 0.5...
Turns out the little empty bullet shell models we far too complicated and 1200 of the little buggers interacting with the environment at the same time was more than any PC could handle.