My timer is being weird and very laggy when run?

Hi everyone!

I was making a timer that would count up by milliseconds (like a speedrun timer) when an event is fired. Here is the local script for that below:

local isRunning = false
local rs = game:GetService("RunService")

function round(num)
	
end

script.Parent.Start.Event:Connect(function()
	if isRunning == false then
		isRunning = true
		local timer = script.Parent
		while isRunning == true do
			rs.Stepped:Connect(function(sec, _)
				local miliseconds = math.floor(sec * 100)/100
				local seconds = math.floor(sec)
				local minutes = math.floor(sec / 60)*60
				if miliseconds < 10 then
					miliseconds = "0" .. tostring(miliseconds)
				end
				if seconds < 10 then
					seconds = "0" .. tostring(seconds)
				end
				script.Parent.Text = minutes .. ":" .. seconds .. "." .. miliseconds
			end)
		end
	end
end)

script.Parent.Stop.Event:Connect(function()
	isRunning = false
	script.Parent.Text = "00:00.00"
end)

When the event fires, so much lag happens that it can barely run, taking about 10 mins to stop entirely. On top of that, there are 4 places for digits (something like this -> 34:45.09.32, the extra ".32" at the end shouldn't be there) in the timer. Why does this lag happen, and why is the extra bit created?

Thanks in advance :)