Why do these parts move slower over time in a loop?

Hi. I'm trying to make a script that creates parts in random positions around the camera, with those parts moving upwards.

I've succeeded in the first part, but when I try to make the parts move up in a loop, they become slower and slower, and become less smooth in a short period of time.

Here's a video showing the problem. Notice how they start off going upwards smooth and fast, but very quickly lose speed and smoothness.

Here's the code:

local debris = game:GetService("Debris")

local cam = workspace.CurrentCamera
local rand = math.random
local num = 25

while true do
	for i = 0, 10 do
		local brick = Instance.new("Part")
		brick.Anchored = true
		brick.CanCollide = false
		brick.Size = Vector3.new(1, 1, 1)
		brick.Parent = workspace
		brick.CFrame = CFrame.new(cam.CFrame.Position + (cam.CFrame:VectorToWorldSpace(Vector3.new(0, 1, 0).unit) + Vector3.new(rand(-num, num), rand(-num, num), rand(-num, num))))
		debris:AddItem(brick, 0.75)
		
		local function moveUp()
			while true do
				brick.CFrame = brick.CFrame * CFrame.new(0, 0.1, 0)
				wait()
			end
		end
		spawn(moveUp)
		
	end
	wait()
end