This kind of thing can't really categorise as a game its more an agility test one that using a space bar over say a touch screen really ruins it, I felt a lot of this game will come down to the different types of space bars out there how many inputs your buffer hold and how fast it will spring back up to neutral. This probably would be fixed by being on a touch screen, the other much more noticeable issue here is that the game dose not motivate me to keep playing.
Have it record your high score that way you always have a viable goal to try and beat, maybe a meter that builds up to show how close I’m getting to beating it as I play. Combine that with some New Grounds leader boards and you've added some strong competition to the game and a reason to keep playing.
Also overhaul the entire aesthetic of this, rather than say black screen with countdown, it’s every time you push the button your character kills a monster or your space ship shoots down something, literally anything at all. You can combine that with say different sprites for every 25 tap intervals and you have some mystery that makes the player want to see what comes next and try (tap) harder to get there, keeping someone playing for more than a minute. There are many things you can do to improve this game and while many will disagree there is no such thing as a bad gameplay mechanic only a poorly executed one and even a game as like this could be great if done right.
So put in the effort and do it right, until then this game is a bad 30 second distraction that comes off as lazy and pointless when its mechanics do have potential with even minor tweaks.