Fast velocity increase but slow decrease with force

Hi!

I need for a sideways 2D space shooter game a spaceship that increases x velocity instandly when a button is pressed but decreases x velocity slowly when the button is released.
I need the actual velocity to be read out of the rigidbody for further calculations (for the enemies to pre-aim at you)..so therefore I think I need some kind of addforce here..

How could this be done?

Thanks
Andy