The frame rate is too low.
Reduce the ammount of exposition for each drawing and check your scene frame rate.
I've seen some animations pulling fluency when animating at 6 or 8 fps, but thats because the poses were very dynamic or the movement worked fine looking slow (one of those was an Oni turning into stone).
Imagine that you're animating three-dimensional shapes instead of moving lines. That will make a world of difference in whatever you animate. Unless you're animating something that looks flat on purpouse.