I've had a dep slot come in for this weekend and need to learn a few and brush up on a few. Was reacquainting myself with the final chorus of Footloose and kept tripping over a few bars. So reverting to first principles I thought 'slow it right down, play to a metronome, gradually speed it up'. As youtube was open I decided to use a drum beat on there and found this wonderful vid. Not having any Apple products about the place I can't try the app itself but frankly this is plenty good enough for me. Just >> to a suitable starting tempo and let it run. The imperceptible increase is just great for getting up to and beyond the speed you need.