You could make it a press/hold thing as well. The speed up thing is less of an issue for me, it's that you don't have control over when you're firing, which makes aiming the projectiles more difficult.
Speeding up would also make it easier to aim since you wouldn't have to lead the shots as much, but you'd also have to adjust the difficulty a bit to compensate. I prefer faster feeling games myself so that may be my personal bias talking :P