Script running very slow and lags game like crazy?

As like the title said. It's lagging very bad and slow, too.

Code:

function _FILLWATER(x, z)
	_CUMULATED = _CUMULATED + 1
	
	for _Y = _Y / _SCALE - 1, -_OFFSET, -1 do
		local _PART = Instance.new("Part")
		_PART.Anchored = true
		_PART.CanCollide = false
		_PART.BottomSurface = Enum.SurfaceType.Smooth
		_PART.CFrame = CFrame.new(x * _SCALE, (_Y * _SCALE) , z * _SCALE)
		_PART.formFactor = Enum.FormFactor.Symmetric
		_PART.Name = x ..'-water'
		_PART.Transparency = 0.5
		_PART.Size = Vector3.new(3, 3, 3)
		_PART.TopSurface = Enum.SurfaceType.Smooth
		_PART.Parent = _BIN
		_ALTDECALS(_PART)
		for i, v in pairs (_BIN:GetChildren()) do
			if v:IsA('Part') and v.Position == _PART.Position and v.Name ~= x ..'-water' then
				_PART:Destroy()
			end
		end

		if _CUMULATED % 32 == 0 then
			wait()
		end
	end
end

The code that actives it:

for _X = 1, _X do
	for _Z = 1, _Z do
		_FILLWATER(_X, _Z)
	end
end