Cute game, but wow is that scroll speed really fast, hahah :)
I timed it with a screengrab, and going frame by frame, it took 0.3 seconds from an enemy's first frame onscreen to impacting the player, at least on my machine. Human reaction time is already 0.25 seconds not even accounting for making decisions and actually physically depressing the key.
I suspect your animation setup or physics implementation may just have a framerate dependence somewhere which might explain why it probably seemed fine on your machine but other people are complaining its too fast.
All in all a great first showing and I hope you keep it up! Thanks for sharing :)