How do I use the same variable in a regular script in a local script?

You have a couple problems:

• You're using a global enabled which means 2+ people can't fire ice spikes at once. To fix this, you need to use a dictionary that tracks whether it is enabled for each player.
• You're having the client modify CFrame & Velocity, which will get cut off due to FilteringEnabled (ie the server won't recognize it; the player will end up seeing what you wanted to happen, but the server won't know about it). To fix this, you need to set up everything on the server. The only thing the server doesn't know is the mouse, which you aren't using. If you were to start using it (so the projectile goes towards where the user's mouse is aiming), you could send those details through the RemoteEvent.

You mention performance problems - if you're sure it's from these scripts, it might have something to do with MaxForce of the BodyVelocity being set to math.huge, or perhaps you simply have too many moving parts in your place.

Improved server script:

---------[IceSpike]--------------------------------------------------
local IceSpikeEvent = game.ReplicatedStorage.MagicEvents.IceSpikeEvent

local enabled = {}
game.Players.PlayerAdded:Connect(function(player) enabled[player] = true end)
game.Players.PlayerRemoving:Connect(function(player) enabled[player] = nil end) -- Prevent memory leaks

IceSpikeEvent.OnServerEvent:Connect(function(player)
    if not enabled[player] then return end
    local torso = player.Character and player.Character:FindFirstChild("Torso")
    if not torso then return end
    local projectile = Instance.new("Part")
    projectile.Size = Vector3.new(2,11,2)
    projectile.BrickColor = BrickColor.new("Pastel Blue")
    projectile.CanCollide = false
    projectile.CFrame = torso.CFrame
    local projectileMesh= Instance.new("SpecialMesh")
    projectileMesh.MeshId = "rbxassetid://3508829528"
    projectileMesh.Scale = Vector3.new(1,0.8,1)
    projectileMesh.Parent = projectile
    local projectileEffect = Instance.new("Smoke")
    projectileEffect.Parent = projectile
    local projectileVelo = Instance.new("BodyVelocity")
    projectileVelo.MaxForce = Vector3.new(math.huge,math.huge,math.huge)
    projectileVelo.Parent = projectile
    projectileVelo.Velocity = torso.CFrame.LookVector * 150
    projectile.Parent = workspace
    game.Debris:AddItem(projectile, 1.5)
    enabled[player] = false
    wait(2)
    if enabled[player] ~= nil then enabled[player] = true end -- If they haven't left the server, re-enable it for them
end)

The client script is the same, except it doesn't change projectile nor projectileVelo.

If you wanted the client to make changes that only the client can see, you could use a RemoteFunction and return projectile so the client knows what to modify.

this is how I did it, when I need it.

put local script and server script in the same model, create a value instance in the model, say number, string, object, vector3, whatever you choose. server script updates the value, local scripts read the value.

You would need to use a Remote Function so that your projectile object yields back to your local script. Like this :

Local Script:

local IceSpikeEvent = game.ReplicatedStorage.MagicEvents.IceSpikeEvent
local player = game.Players.LocalPlayer
local mouse = player:GetMouse()

mouse.KeyDown:connect(function(key)
    if key == "e" then
        IceSpikeEvent:FireServer()
        local projectile = game.ReplicatedStorage:WaitForChild('RemoteFunction'):InvokeServer()
        projectile.CFrame = CFrame.new(player.Character.UpperTorso.Position) --help
        projectile:FindFirstChild('BodyVelocity').Velocity = player.Character.UpperTorso.CFrame.lookVector * 150
    end
end)

Server Script:

---------[IceSpike]--------------------------------------------------
local IceSpikeEvent = game.ReplicatedStorage.MagicEvents.IceSpikeEvent
enabled = true

local remotefunction = Instance.new('RemoteFunction')
remotefunction.Parent = game.ReplicatedStorage


IceSpikeEvent.OnServerEvent:connect(function()
    if not enabled then return end
    local projectile = Instance.new("Part")
    projectile.Size = Vector3.new(2,11,2)
    projectile.BrickColor = BrickColor.new("Pastel Blue")
    projectile.CanCollide = false
    local projectileMesh= Instance.new("SpecialMesh", projectile)
    projectileMesh.MeshId = "rbxassetid://3508829528"
    projectileMesh.Scale = Vector3.new(1,0.8,1)
    local projectileEffect = Instance.new("Smoke", projectile)
    local projectileVelo = Instance.new("BodyVelocity", projectile)
    remotefunction.OnServerInvoke = function () return projectile end
    projectileVelo.MaxForce = Vector3.new(math.huge,math.huge,math.huge)
    projectile.Parent = workspace

    game.Debris:AddItem(projectile, 1.5)
    enabled = false
    wait(2)
    enabled = true
end)

There you go, I hope I helped you :)