Thanks for playing! You beat my "high score" of 10k. I'm not using any object pooling, which I probably should've (altough from what I know Godot already handles it decently). I never had any issues on the standalone (3000+ grains out) and performance seemed good on web too so didn't bother spending time optimizing it.
Could be that the sheer amount of allocation and deallocation is causing performance drops, even when there isn't much grain placed at a point in time. With that said I've got a powerful computer (RTX 3080) so that's probably the reason.
I'm happy to hear that you could play the game to wind down after a long day!