multiplayer - how should a server handle client actions?

So I've just started trying to make a multiplayer pong game and I have read a lot of different ways to code the connections. However, I read somewhere that you should have the inputs sent to the server queue up and then the server just processes it at fixed intervals. However, if this is done, how does the server know, process, and send accurate data based on the time the action was sent? Because if the data is just queued up, if something happens like 100 ms after another but the server processes them at the same time, wouldn't that mess up the synchronization of the game? Should the server instead be coded to handle the client actions immediately even if it costs a lot of cpu etc in larger games?