The normal mode is pretty fun but its kinda easy, so after 40 points I got bored and tried the slow mode. The slow mode just makes you spam clicking and its a bit annoying. My practical feedback:
Remove the slow mode and add more to the gameplay. Maybe add a delay so that the player needs to wait some time before changing directions again. You can also make it so there will be more ink stains on the screen to make the game less empty, but also add things you need to avoid to balance the difficulty. You can also add objects that will effect the movement of the ball, I'm just throwing ideas to spice up the game but you should definitely think what you want to add by yourself because the story is so creative.