[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?