At its highest point
Assuming that gravity is the only force acting on a projectile ( ie no air resistance etc.)
when a projectile is released with a given velocity at a given angle, the velocity can be expressed as two components ( ie resolved).
The vertical component and the horizontal component
1) since gravity is the only force acting the horizontal velocity is constant and is unchanged throughout the motion. This component only affects its range
2) the vertical component is responsible for the vertical motion of the projectile. Gravity acts on this component and initially it will slow the projectile down, eventually to