Yes this is exactly THIS upgrade ;) but actually it was not necessary since the first of the leaderboard made his score without it
By the way yes, you're absolutely right, the spawning rate is too high at some point and this is why the game looks so weird after some time. I will definitely check out how to fix that ! I will also see how to make the difficulty a bit less brutal because basically after the third wave this is almost impossible if you do not have a decent build
Thanks again for your comments ! This is the first time I make a 2D game and this is such motivating to have honest and detailed reviews like that. I will never thank you enough for that :D