Comment below rating threshold, click here to show it.
I bring this up not as a hack/fix, per se, but rather because it might help the developers.
When a match launches, a new process is started that corresponds to the match that was created. I've noticed that if I run the "renice" command and set the priority for that particular process to the highest possible (-20), I no longer get massive slowdown during teamfights. It still dips a little, but not dramatically. Previously, it would drop to about 5fps, now it won't generally go below 20fps.
I don't want to give a detailed explanation of how to use the renice command in this situation, even though it isn't secret, because I know the last thing the developers want are a bunch of users doing workarounds when they are trying to fix bugs.
I did, however, find this to be interesting, and hopefully it will help the devs...