Im creating a game with alot of projectiles, its sort of like a bullet hell game but combat styled.
I currently use cframing to move the projectile. But still after around 80 projectiles per second the server starts to lag.
(the lag is more significant with body velocity.)
I use the PartCache module to optimize projectile spawning, however the hard part is the movement.
I used client hitboxes to minimize the server workload.
Yes fastcast is designed for performance and it does not have any issues with lagging projectiles. Some of the biggest games on Roblox actually use fastcast.