I'm adding a flying rock effect to a projectile and it works, but goes super slow. I'm assuming its the Velocity so I have posted my velocity script here.
local bodyvelocity = Instance.new("BodyVelocity")
bodyvelocity.MaxForce = Vector3.new(math.huge,math.huge,math.huge) * 40
bodyvelocity.Velocity = Vector3.new(math.random(-13, 25),math.random(-13, 25),math.random(-13, 25))
bodyvelocity.Parent = FX
(FX is the flying rock)

2 comments
Just mess around with the 3 values of Vector3 that set the velocity.
Those 3 values are for the rocks to go different directions while rising. I can't or it wont look like they are different sizes, etc.