Why do I have 10 fps in team fights

Can someone explain to me wtf happend with game optimization? In every game I have, I have 30 fps and if I'm lucky - 40 And in team fights I lose half or more fps that I have normally. Some time ago I had stable 60 that occasionally went to 50 but it was still playable and in team fights I had around 40 fps more or less. Because of this I lose some of fights that I should won but because of that 1 sec of 1 fps is able to kill me. And no, this isn't because of my pc.