Are you using listen or dedicated server?
How many projectiles are spawned in your game in 1 minute on average?
How many projectiles are alive in the same time in 1 minute?
Have you heard about object pooling and projectile pooling in general?
When you tried replicated projectiles, did you also replicate their movement?
Tell me about how the projectile are supposed to be working: spawn with muzzle flash effect at the tip of the character’s weapon and then spawn hit FX when projectile hits something?
Tell me about how you deal damages, is it when the server’s projectile is hitting something? What system then takes care of the damage?
When do you destroy the projectile?
I think it’s perfectly fixable, we just need to think about this and see what are your needs and how we handle them.
Also it looks like the 4.14 version of Unreal has news about networking and netcode in general.