Data Store Efficiency?

local datastore = game:GetService("DataStoreService"):GetDataStore("Stats")

game.Players.PlayerRemoving:connect(function(player)
player:WaitForDataReady()
    local key = "user_" .. player.userId
  datastore:UpdateAsync(key, function(oldValue) -- We don't wanna use SetAsync we don't want any chnaces of data lose anyway
      local a = player:WaitForChild("leaderstats"):GetChildren()
 for i= 1,#a do -- For every number of children 
        DataStore:SetAsync(a[i].Name, a[i].Value) -- Saves all children values
        end
      end)

So first of all, when the player removes, it Saves it username and stats, I added comments on some. Now when player joins :P

local datastore2 = game:GetService("DataStoreService"):GetDataStore("Stats")
game.Players.PlayerAdded:connect(player)
wait(20) -- We don't want to reach maximum request for datastore so the player waits 20 seconds before loading
player:WaitForDataReady() -- Waiting for player
  local key = "user_" .. player.userId
 if  DataStore:GetAsync(key) ~= nil then -- If they is not equal to nil it will get and load stats
local a2 = newplayer:WaitForChild("leaderstats"):GetChildren() -- Make sure that the script doesn't load before leader stats
for i = 1, #a2 do -- Same as old one
 datastore2:GetAsync(a2[i].Name) -- Brackets for as2[i] means for every one or is equal to i
end
end
end)

You can put both scripts in the same script, This should work. Good Luck HexC3D