I tried your game and it ran smoothly. I like it. I reached level 6 score 27, at that point the coin and my cursor felt like repelent magnets .
In my opinion you gotta add more tweenings.
I don't know if the landscape is always the same. The one for me was a river in the forest and a car, so maybe you should add:

• car's light flashing
• shooting stars
• dark shadows in the forest