Suggest this perhaps,
@tobiasvl. Puzzle could be made harder if you do not press keystroke within TIME WINDOW.
That is, if you wait TOO LONG then that is also considered an invalid stroke. Player must learn and "feel" the correct answer, but I would make the final bar exactly twice of each, in fact you could do that the whole way across.
Each bar, there are 10, yes ? Might do this where each bar is doubled in time exactly.
level 1=0.0098
level 2=0.0196
level 3=0.0392
level 4=0.0784
level 5=0.1567
level 6=0.3135
level 7=0.627
level 8=1.2539
level 9=2.5078
level 10=5.0156
In fact you could have levels of play where the first level has no window (or 5.0156), the next 2.5078 mark, then half, then half of that, the differentiating stroke between times per level. So the final level would be level 10 and you must time your strokes exactly within 0.0098 seconds to the mark.
Even though people might guess what the point is for the first level by just delaying, the next level would require them to have a keener understanding of the timing involved.
You might give them 9-lives too. If all 9 are used up, they must start again from level one.