That's exactly the point -- you need to try to keep up. This slight frustration is the part that makes you get better ;)
If you don't press space, you're violating the idea of words. If you press space, the sentence you've constructed in your head is broken. Either way, it slows me down and frustrates me too much to play :)
The original study used only a timer, and had the same frustration which you speak of here.