As you go down in speed right now, the time at which the note stays on screen goes up exponentially (look at the graph), so I suggest counteracting that distribution so that the perceived time would be more linear.
Graph made from formula(s) found here: t/367649
(Vertical axis: time the notes appear for, Horizontal axis: Fixed speed)
Graph made from formula(s) found here: t/367649
(Vertical axis: time the notes appear for, Horizontal axis: Fixed speed)