local Players = game:GetService("Players")

-- Memoization: since these results are rarely (if ever) going to change
-- all we have to do is check a cache table for the UserId.
-- If we find the UserId, then we have no work to do! Just return the name (fast).
-- If we don't find the UserId (cache miss), go look it up (takes time).
local cache = {}
function getUsernameFromUserId(userId)
	-- First, check if the cache contains the name
	if cache[userId] then return cache[userId] end
	-- Second, check if the user is already connected to the server
	local player = Players:GetPlayerByUserId(userId)
	if player then
		cache[userId] = player.Name
		return player.Name
	end 
	-- If all else fails, send a request
	local name
	pcall(function ()
		name = Players:GetNameFromUserIdAsync(userId)
	end)
	cache[userId] = name
	return name
end

print(getUsernameFromUserId(269323)) --> Ozzypig
print(getUsernameFromUserId(269323)) --> Ozzypig, but this one returns much quicker
print(getUsernameFromUserId(261)) --> Shedletsky