[Tower Defense Game] How would I make an efficient tower aggro system?

So I recently made a game that involves using towers to kill enemies and it locks on to enemies within it's range. However the function that does this seems to cause a lot of lag when it does, and in order for me to check if an enemy is near, I have to use a 'while true do' that goes through every enemy in the enemy folder and checks if it's magnitude from the tower is less than the towers range. I think there is a more efficient way to do this, but I'm not sure. Here's a couple links for images.

https://gyazo.com/80f223c7f11938711bc7488978b293ac

Anything within the blue circle it will shoot. However this is how I made it function.

local function getNearestEnemy()
    local lowest = 500
    local nearObj
    local children = #enemyFolder:GetChildren()
    for i, v in pairs(enemyFolder:GetChildren()) do
        local rootPart = v:FindFirstChild('HumanoidRootPart')
        if rootPart ~= nil then
            local health = v:FindFirstChild('Health')
            if health ~= nil then
                local groundPos = hitbox.Position - Vector3.new(0, hitbox.Size.Y / 2, 0)
                local mag = (groundPos - rootPart.Position).magnitude
                if health.Value >= 1 then
                    if mag <= range.Value then

                        if mag < lowest then
                            nearObj = v
                            lowest = mag
                        end

                    end
                end
            end
        end
        if i == children then
            return nearObj
        end
    end
end


spawn(function()
    while true do
        wait(.1)
        if enemy == nil then
            local nearestEnemy = getNearestEnemy()
            if nearestEnemy ~= nil then
                enemy = nearestEnemy
            end
        else
            if enemy:FindFirstChild('HumanoidRootPart') == nil then
                enemy = nil
            else
                attackEnemy()
            end
        end
    end
end)

Some variables that you see may not be included in the code block I provided. There is a big issue with how my towers run though. As you can see, every .1 seconds it will check for nearby enemies. That means every second, it has gone through the enemy folder, 10 times. Which I think can easily be seen as a cause of lag. What I want to do is trigger a towers attack when an enemy gets close enough. How might I go about changing this into that type of system?