I think it was "explosions leave poison clouds" with "2x explosions" upgrade that lead to it being as bad as it was, combined with several +duration upgrades to clouds. I had a frankly silly number of clouds that lasted long enough to still cover the screen on the next wave. I had a run that looked a lot more visually chaotic after, that only pushed my 7600x to like 70 degrees give or take. Whereas the first one was pushing it all the way up to 83, which is basically where it caps out with my cooling setup.