delay.lua

local RunService = game:GetService("RunService")

local delay do
	-- uses a sorted singly linked list (queue) to achieve O(1) remove operations and O(n) for insert

	local first -- the initial node in the linked list
	local connection -- the Heartbeat `RBXScriptConnection | nil`

	function delay(seconds, resolve)
		assert(type(seconds) == "number", "Bad argument #1 to delay, must be a number.")
		-- If seconds is -INF, INF, NaN, or less than 1 / 60, assume seconds is 1 / 60.
		-- This mirrors the behavior of wait()
		if not (seconds >= 1 / 60) or seconds == math.huge then
			seconds = 1 / 60
		end

		local startTime = tick()
		local endTime = startTime + seconds

		local node = {
			resolve = resolve,
			startTime = startTime,
			endTime = endTime
		}

		if connection == nil then -- first is nil when connection is nil
			first = node
			connection = RunService.Heartbeat:Connect(function()
				local currentTime = tick()

				while first.endTime <= currentTime do
					first.resolve(currentTime - first.startTime)
					first = first.next
					if first == nil then
						connection:Disconnect()
						connection = nil
						break
					end
					currentTime = tick()
				end
			end)
		else -- first is non-nil
			if first.endTime < endTime then -- if `node` should be placed after `first`
				-- we will insert `node` between `current` and `next`
				-- (i.e. after `current` if `next` is nil)
				local current = first
				local next = current.next

				while next ~= nil and next.endTime < endTime do
					current = next
					next = current.next
				end

				-- `current` must be non-nil, but `next` could be `nil` (i.e. last item in list)
				current.next = node

				if next ~= nil then
					node.next = next
				end
			else
				-- set `node` to `first`
				node.next = first
				first = node
			end
		end
	end
end

delay(3, print)
delay(5, print)
delay(4, print)
delay(2, print)
delay(1, print)