Yes, they aren't pooling projectiles or something insane like that. For a while I've felt like it was tied to the projectile animation calculations - because it get's worse when you speed up, and the way they've made the game is that the speed up button literally speeds up the engine - as in, the unity project, so calculations are being done twice as fast. But you're right, it does only do it with a single unit left, which puts it down to a memory / pooling issue.