Trying to use raycasting to make a minimap, but having some lag problems. Help?

The best method that I have found of how to make a minimap in Roblox is by using raycasting. I made a script where rays are shot from the sky down to the map; the first part the ray lands on, a tiny pixel is added to a 500x500 frame with the same color as the part. This is the script I came up with.

function castRay(width, length, startLength, dim) --Width is the length of the map in X, length is the length of the map in Z, startLength isn't important, and dim is how big each ray is.
    local player = script.Parent.Parent.Parent.Parent
    for i = startLength, length do
        for j = 0, width do
            wait(.0000000000001) --Even with this wait, it still takes AGES
            local point = Instance.new("Part", game.Workspace) --Creating a part to represent points on the minimap
            point.Name = i+j+1
            point.Size = Vector3.new(dim, dim, dim)
            point.Position = Vector3.new(560.5-j*dim, 300, 942.5-i*dim)
            local ray = Ray.new(point.CFrame.p, (Vector3.new(point.Position.X, -10, point.Position.Z)-point.CFrame.p).unit
            *400) --Creating a ray that aims directly down onto the point
            local part, position = workspace:FindPartOnRay(ray, player.Character, false, true) --Locating the part the ray lands on
            if part ~= nil then
                local dot = Instance.new("Frame", script.Parent) --Creating a new point for the minimap
                dot.Name = point.Name
                dot.BackgroundColor3 = Color3.new(part.Color.r, part.Color.g, part.Color.b) --Making the color the same color as the part
                dot.BorderSizePixel = 0
                dot.Size = UDim2.new(0, 1, 0, 1)
                dot.Position = UDim2.new(0, j, 0, i)
            end
            point:Destroy() --Destroy the part to reduce lag
        end
    end
end

castRay(499, 499, 0, 4) --Activating the function. The minimap is 500 by 500.

The problem is that this makes my computer lag a whole lot. It's also a very time consuming process. I tried making the dim value 10 instead of 4, which would reduce the resolution of the minimap thus reducing the lag and amount of time required, but then the outcome just looked messy.

I want to be able to make a minimap with good resolution without having to deal with lag and preferably without having to deal with hours of waiting. How can I accomplish this? The entire map is 2000 by 2000 studs by the way.