Slower Projectiles is a support gem. It reduces the speed and increases the damage of supported projectile skills. It won't affect how far arrow projectiles travel, since arrow projectiles have a fixed travel distance of 120 units.[1] However, it will still affect the travel distance of projectiles from skills such as Freezing Pulse, which have a lifetime rather than a fixed travel distance.

Slower Projectiles can support Ball Lightning, which causes the projectiles to deal more total hits/damage because enemies would be in the area of effect for longer before the projectile passes by them. At level 20, Slower Projectiles would result in approximately 9 Ball Lightning hits to an enemy instead of 6, other factors being equal.