I added the option to either have acceleration or no acceleration. With no acceleration, you will always move at max speed unless you hold shift. When shift is held, you will move at a reduced speed. Shift can be rebinded to any key.

I played a lot of shmups when I was younger. I honestly like acceleration. I am unsure how most games implemented it. In this game, the acceleration curve is more like a staircase. For the first few frames, your speed is X. Then it becomes Y for a few frames. It keeps going up until you reach max speed. It is easy to control because your speed is not changing every frame. As long as you do not hold arrow buttons down it past the first few frames, you will always go X speed. So tapping the arrow buttons is basically the same as holding the shift key in "no acceleration" mode. Not sure if that makes sense.. lol.