Based on your previous topic, it seems that you've added a higher level of drag to help the object slow down naturally.
Since you've increased this, it's going to take longer for the object to "overcome" that drag and reach its desired speed through addforce.
I would either :
A) increase the force to compensate for increased drag (although this might return you to your previous issue)
B) During the state that is active when the key is being pressed, add a "set property" action and change the drag to its original, lower value.
During the state that is active when the directional key is released, add a "set property" action and change the drag to a higher value which corresponds with the desired reduction in speed.