So looking back on that Phase v Antis game, there's only like 2 enemy types (the normal ones and then the fat ones with a daki). As the game progresses, it doesn't add more enemy types (it just removes the normal guy altogether) and instead it just increases the rate of the fat ones spawning. However with enough upgraded Sakanas, it doesn't matter how many it spawns at a time because 3-4 Sakanas are enough to 1-2 shot them and the AoE cleaves them all. The maker didn't expect people would get to 50k+ coin rounds and there's probably no upper limit to the spawning, so eventually it just throws out so many it lags the whole thing. Pretty good for a gamejam though because they didn't have the time to playtest this shit.