Changing speed is different for different things depending on their current speed and size, or their mass. 

To understand this, we use something called momentum.

MOMENTUM IS WHAT WE GET WHEN WE MULTIPLY THE MASS OF AN OBJECT BY ITS VELOCITY.

A large semi-truck traveling at 60 mph will have more momentum than a small car at the same speed.  A person walking will have less momentum than a person running. It will take more energy for a person to slow down when they are running compared to when they are walking because they have greater momentum.