Enhancing the speed of the animation - setDelayPerUnit

I would like to decrease the delay between frames of an animation. However no matter how much I decrease the value in the setDelayPerUnit the animation does not speed up.

Here is what I am doing:

 Animation* walkingAnimation;
 ...
walkingAnimation->setDelayPerUnit(0.0025f); //set it to 25 milliseconds

Is there any suggestion on how to speed up the animation ? IF I insert the images in here
and set the speed to 25 milliseconds the animation is perfect. My question is how I can speed up the animation of the frames ? Is there a reason why this animation is slow ?