It was fun until I unlocked the 10 gold coins, which take 5 taps/swipes to collect each one. They also have collision mechanics that scatter them all over the screen. It is a greater task to collect gold now than it is to fight the UFO.
Anyone who wants to push far, I suggest sticking with 9 gold per kill.
Edit: The huge coins seem to be attached to the 8 ball. Removing it solved the problem for now at least. They are probably 20 gold coins. Hard to tell when the turtle collects the same time I do.