When a ball is struck by the snooker cue, another ball or a cushion, it experiences a large and brief force (indicated by a red arrow showing its direction and size) which causes a sharp change in its velocity.
A moving ball also experiences another more gentle force - the force of friction which acts in the opposite direction to its velocity, slowing the ball down.

The impact of balls with the cushion (and also of balls with balls) is not completely elastic; they are slowed down by the collision.
One sign of this is that the force on a ball from a glancing impact with the cushion is not entirely perpendicular to the cushion; there is a component of that force in the opposite direction to the ball's parallel  movement to the cushion - slowing down the ball's movement in that direction.
The ball's perpendicular speed to the cushion is also reduced by the collision.