The scenes and drawing must keep up with the narration.  There are 3 ways to slow the drawing down.

1.  Remove elements from the scene.

2.  Make the narration even slower.

3.  Keep the narration and scenes as is and draw less.  To do this, we would need to bring in the elements of the video is other ways and not draw them.  For example, fly them in or pop them in.

If we need to do option 3, we would need to know exactly what scenes your client thinks “moves too fast” so we can see what we can do.