Jittery ball when on low fps

[lilac falcon]

-- Client
local function UpdateProjectile(Index, CurrentPosition, A, Velocity, dt)
    local CurrentDodgeball = Dodgeballs[Index]
    if CurrentDodgeball then
        local Time = tick() - dt
        local SteppedVelocity = Velocity + A * Time
        local SteppedPosition = CurrentPosition + SteppedVelocity * Time
        CurrentDodgeball.Position = SteppedPosition
        
    end
end

-- Server
local Time = tick()
Remote.UpdateProjectile:FireAllClients(index, CurrentPosition, A, Velocity, Time)

it shouldn't be jittering, i dunno why it is

local Conversion = 196.2/9.8
local Velocity = (dest - start).Unit * force * Conversion
        
local A = Vector3.new(self.Wind.X, self.Wind.Y - self.Gravity * 9.8, self.Wind.Z) * Conversion -- wind is 0
        
local T = 0

while true do
    local dt = task.wait()
    T += dt
            
    -- v = v0 + at
    CurrentVelocity = Velocity + A * T
            
    local ProjectilePosition = Vector3.new(
        start.X + Velocity.X * T + 0.5*T*T,
        start.Y + Velocity.Y * T + 0.5*A.Y*T*T,
        start.Z + Velocity.Z * T + 0.5*T*T
    )
    RayResult = workspace:Raycast(CurrentPosition, ProjectilePosition - CurrentPosition, self.Params)
            
    local Time = tick()
    Remote.UpdateProjectile:FireAllClients(index, CurrentPosition, A, Velocity, Time)
            
    CurrentPosition = ProjectilePosition
end

[timid cedar]

Don't calculate elapsed time with the return value of task.wait(). It only returns how long it waited for meaning the elapsed time will be off by the amount of time it took your code to run.
Use os.clock(), it returns elapsed time since server start

Don't use single character varaible names, it makes your code hard to read

[lilac falcon]

alr

[timid cedar]

The calculation of A can be simplified to

local A = (self.Wind - Vector3.yAxis * self.Gravity * 9.8) * Conversion

[timid cedar]

You probably shouldn't be firing an event every frame for the ball position. Since the ball's trajectory is deterministic you can simply tell the client where it starts and what time and then the client can simulate it itself

That will remove any jitter from network lag

You could also use unreliable remote events if you don't want to simulate it synced on the clients. Unreliable remote events aren't granteed to be recived and aren't garunteed to be in the order they were sent. They are useful for things that need to be updated frequently but don't suffer if a single message is lost. They are faster.

[lilac falcon]

so either work with unreliable remote events (which can be sent in the wrong order or may be lost) or have the calculations along with the visualization on the client?

[timid cedar]

Yes

which can be sent in the wrong order or may be lost
A lost event doesn't matter, to handle out of order events just send an index along with any event and when an event is received drop any that have a lower index than the highest index you've received so far.