the trigger to pollution level that high that it's clear it's a mega base = tons of enemies will swan = pc will slow down.
From that point, while the game still keep track how much pollution it absords, how many bitters it will create, how they will meet and then go to attack, nothing happens.... why? Did you play rampant with underground attacks?
well my ideea is similar but different.
Ideea is that after a x value of pollution we create, the enemies to did
And as a program, the ideea is simple to create an excuse why no enemies are being visible. Because the ideea of tunel is that they come out one by one, well, in a way. Ideea is that no need to spawn 100+ bitters, since more or less the row attacking, if killed, there is another one behind them, continuing this over and over again. Meaning that bullets will become once again critical to have since flames might or might not, be enough since they can't attack everything since not everything is yet outsite. Also as the bitters try to come out, the "start" will get wider and wider. Like this we reduce the impact on pc since the enemies don't exist in game until they "pop out" and via the "tunnel system, one behind the other" not all will be on the map at the same time, while, due to being underground = immune to damage, means that they will be more dangerous then ever with no direct way to attack them or prevent them except via walls. And while we can use walls as a way to get them out of the ground, what do you intend to do after? manually attack them, every last one of them? let the eneies attack and destroy the walls before going to the real frontier for flametrower to be able to kill as many as possible? See... this is why i said, the walls to act as a "need to get out". Because if no wall was hit (since the bitters that went up destroyed the walls) most of the remaining enemies are still hidden, untouched. Meaning it's more crucial as ever to protect the walls and of course, to create as much pollution as possilbe to prevent them from coming to close to the base. While also any pollution reduction will be even more important.
Bonus: atomic bomb's ground = they will come out BUT, the nuclear scorn to have a "hp like" count, so after some time, it will dissapear. Meaning we can create an artificial wall, but will cost a lot of bombs, meaning finally a good use for the tons of uranium we have.
Bonus 2: regarding flamethrowers being to op (since they just keep going in the same exact spot), the moment the bitters come out, due to being covered in earth, they have 100% fire resistance, and this stays for a good few seconds (after all, if you do enough infinite research, even at 99% fire resitance they might instantly get killed.
so again, the whole point of my suggestion is to have an excuse to NOT have every last enemies visible both via delaying when to attack, as well as how they will "show up" (aka not everyone at once) while at the same time, pollution remains the trigger and will heavely affect enemies attack like per usually. However what will change is that we fianlly don't experience FPS/UPS drops OR force to reduce the resolution or similar