Hi, there are several problems with lag in the game, especially when there are a lot of enemies on the screen, largely due to the Game Boy hardware which was made as the basis for the game.
This issue of the animation making the game slow is a little complicated, I purposely made the animation slow at the beginning of the load and increase the speed when the player reached the maximum load. I don't know if when the player reaches maximum speed, it causes the game to slow down because of the fast animation. I'll do some testing and release a version with less lag if I can solve it.