Case Opening script goes slow at the end?

Hey I've recently found a case script that works and have a few questions about it.

1. Why does the case opening slow right down at the beginning
2. Why does it sometimes land in between items and gives you another item

If you can answer my questions or perhaps have a better way to do this, I will greatly appreciate it

Code is below

local RarityI = {  -- lower the rarer
	["Common"] = 5,
	["Uncommon"] = 3,
	["Rare"] = 2,
	["Legendary"] = 1
}

local CrateInUse = false

local function opencrate(Chance)
	if CrateInUse == false then
		CrateInUse = true
		script.Parent.Unboxing.Inner:ClearAllChildren()
		local Items ={} -- random Items
		local frametimes = math.random(20,50)
		local rewardbutton --win item
		--making the itemlist
		for _, Item in pairs(game.ReplicatedStorage.Items:GetChildren()) do
			local RarNumber = RarityI[Item.Rarity.Value]
			local Times = math.floor(RarNumber+Chance) -- gives bigger chance on better Items
			for num = 1,Times do
				table.insert(Items,Item)
			end
		end		
		
		math.randomseed(tick()) --makes a seed for that second
		-- generating images
		for num = 1, frametimes+5 do
			local Item = Items[math.random(1,#Items)]
			local newFrame = script.Parent.Unboxing.SampleItem:Clone()
			newFrame.Visible = true
			newFrame.Parent = script.Parent.Unboxing.Inner
			newFrame.Icon.Image = Item.Image.Value
			newFrame.ItemName.Text = Item.Name
			newFrame.Position = UDim2.new(0,100*(num-1),0,0)
			newFrame.AnchorPoint = Vector2.new(0,0)
			if num == frametimes then
				rewardbutton = newFrame
				print (rewardbutton.ItemName.Text)
			end
		end
		for num = 1,(rewardbutton.Position.X.Offset - (255+math.random(-80,80)))/15 do
			for i,v in pairs(script.Parent.Unboxing.Inner:GetChildren())do
				v.Position = UDim2.new(0,v.Position.X.Offset - 15,0,0)
				if v.Position.X.Offset <= -100 then
					v:Destroy()
				end
			end
			--script.Tick:Play()
			wait(0.01 * 1.05 ^ (num-105)/2)
		end
		print(rewardbutton.ItemName.Text)
	end
end

opencrate(0)