2 Answers

The speed you speak of is "muzzle velocity" that is , the speed of the projectile  as it leaves the muzzle and NOT the rate at which the projectile travels after leaving the muzzle.

 This rate of travel reduces as Jack Large said, so after the projectile has decreased it can be expressed as seconds per second in relation to this decrease in speed.

 A rifle projectile may have a muzzle velocity of 1800 feet per second but depending on atmospheric conditions, wind, gravity ,rain etc the speed may reduce at a rate of say 10 feet per second per second.

By projectile of coures I am referring to the "bullet" in laymans terms.

Because everything in motion that is propelled laterally slows down over time because of gravity.

Things that are dropped, speed up over time until they reach terminal speed, because of gravity.