To be honest, I haven't used it. [sad]

But my thought was more along the lines of not fast songs, but slow ones. Think of a languid, say, Nat King Cole tune. Only tagging one line at a time could conceivably make the scroll not match the singing. But I've done no testing.

Highly syncopated singing could throw a wrench in the works, too. I'm not sure I have a good example for this. Maybe some Ella Fitzgerald or Rosemary Clooney stuff?