Is there a way to make parts instance faster?

I was working on terrain generation when a problem appeared: The parts generate really slow. I tried Instancing MeshParts and other types of parts but it still ran slow. Then i tried the real time CSG and it seems to reduce the lag but not improve the speed.

Here is the code where i used csg:

local Octaves = 12
local Lacun = 2
local Gain = 0.25
local partstab
function fBm(x, y, z, octaves,lacunarity,gain)
    local amplitude = 1
    local frequency = 1.5
   	local sum = 0
    for i = 1,octaves do
		local xf = 1 * x / frequency
		local yf = 1 * y / frequency
		local zf = 1 * z / frequency
        sum = sum + amplitude * math.noise(x * frequency, y * frequency, z * frequency)
        amplitude = amplitude * gain;
        frequency = frequency * lacunarity;
	end
    return sum
end 
local mapScale = 300
local mapHeight = 200
local waterLvl = 10
math.randomseed(tick())
local mapScale = 250
local mapHeight = math.random(10 , 200)
local waterLvl = math.random(10 , 20)
local seed = math.random(1,10e6)
local smoothness = math.random(10 , 60)
    for x = 1, mapScale do
        for z = 1, mapScale do
			partstab = {}
			for _,v in pairs(workspace.Parts:GetChildren()) do
			table.insert(partstab , v)
			end
			if workspace.Parts:FindFirstChild("Part") then
			local newunion = workspace.Parts.Part:UnionAsync(partstab)
			newunion.Parent = workspace
			for _,ae in pairs(partstab) do
				ae:Destroy()
			end
			end
            local height = fBm(x/smoothness, z/smoothness, seed , Octaves , Lacun , Gain) --Find out the height 
            for y = 1, (height*mapHeight)+10 do
                local density = fBm(x/smoothness, y/smoothness, z/smoothness, Octaves , Lacun , Gain) --Find out the density
                if y > waterLvl then
                    if density* -5 >= 0 then
                    	--workspace.Terrain:FillBall(Vector3.new(x * 2,y * 2,z * 2) , 5 , "Grass")
						local p = Instance.new("Part", workspace.Parts)
						p.Anchored = true
      				  	p.Size = Vector3.new(10,10,10)
        				p.CFrame = CFrame.new(x * 10,y * 10,z * 10)
						p.Color = Color3.fromRGB(0, 100 , 0)
                    end
                else
				--workspace.Terrain:FillBall(Vector3.new(x * 2,y * 2,z * 2) , 5 , "Grass")
				local p = Instance.new("Part", workspace.Parts)
				p.Anchored = true
      			p.Size = Vector3.new(10,10,10)
        		p.CFrame = CFrame.new(x * 10,y * 10,z * 10)
				p.Color = Color3.fromRGB(0, 100 , 0)
            end
        end
    end
wait()end

Now i dont know. Games like Minecraft or terraria generate the ground in 5 seconds(Even though terraria is 2d i dont think it would generate terrain much slower). I understand that those games were made from nothing and not game developing platform but is this an issue for roblox?