staticx | Create static executable from dynamic executable | Build Tool library

I am building a maze game based on this tutorial. I successfully got the player rectangle to continue moving as long as you hold down the arrow keys. When you first start the game, the animation is really nice and fast, but it seems that after playing the game for a few seconds the animation gets slower and slower. Can anyone help me figure out why this is happening?

I have created a code snippet, but unfortunately it doesn't work correctly because of a cross-origin error caused by the maze image I am using.