Gravity doesn’t have a speed. It creates an acceleration due to force. The bullet is accelerating over a great distance until it reaches terminal velocity (where the air resistance counterbalances any gains in acceleration).

The easiest way to think about it is in terms of conservation of energy. All the kinetic energy the bullet has at the muzzle is converted into gravitational potential until velocity hits zero. At that point, gravitational velocity starts getting converted directly back into kinetic energy. If not for air resistance, the bullet would reach the ground at roughly muzzle velocity. However, air resistance on the way up zaps some of that energy and presents a velocity barrier on the way down.