Slow In and Slow Out

I think this animation is ok, the over all motion is correct. But the animation is too quick for it to be an example of a good slow in and slow out animation. I found it difficult, to slow down the movement of the spheres at each end when they went to there peak stage of there movement. 

2D animation of a Tennis ball and a Canon ball rolling down some stairs. The Canon ball is more heaver so it wouldnt bounce as much, but the Tennis ball is quite bouncey compared to the Canon ball, because of there weight. I could improve this if I made the Tennis ball more bouncey, and slow the animation down, and probably make the Canon ball move a bit slower compared to the Tennis ball.