I was doing the advanced Plague Star bounty, got to the end, died, then got up and left to go take care of something. It was just me in the mission, laying there, being dead. I come back maybe eight or ten minutes later to a field so full of infested I was dropping frames. Cleared alot out with a necramech and realized the kills were not being counted at all. At some point, I couldn't even hit them because the weapons wouldn't register anything as a target, beams and bullets went right through them. At least the framerate returned. Decided to simply fly out and return to Cetus. Thankfully got to keep the rewards up to that point!
I have a screenshot of what I returned to here: https://imgur.com/gallery/j3ghez4 There were more off-screen to the right and below. It was crazy.
I would highly recommend putting some kind of spawn cap for the end event because the game definitely did not appreciate having so many entities around.