Why is the the graphics so plain? Is this your first game? The game lags a bit, but not overly often on my PC, a 1.8ghz x2, 2GB of RAM with Geforce 7000 Graphics card. That is likely because it was made with Game Maker, I've had that issue with other games created using that.
Have you thought of creating games using a real programing language? It is easy now with game engines such as Flat Red Ball. You may want to look at http://www.flatredball.com/, it could help you with your future in game creation.
Your game could use some breaks, it gets a bit tedious as it is. This type of game is usually broke into 'waves' or 'levels'. Some put bonus rounds between every few levels. Also a brief game play manual posted in the description could be helpful.