Yep, that seems to have fixed it! It was always hovering around the 55fps for me, even with all the orcs surrounding me.
The only time it went lower, was 45fps when I had all of the orcs around me, and I killed about 10 in one go. (I guess all of those animations simultaneously), but it recovered after less than a second back to 60fps.
So, I'm guessing it was the OnTriggerStay? I honestly don't see the point of that action - it's so bloody expensive, that it's unusable for any type of gameplay!!
Btw, there's a weird jittering of the player character sometimes, when walking. That's new, but I guess minor and easy to fix.