Yes, even if you have already checked in online you need to queue for security. However, if you are travelling with hand baggage only and your airline allows it, you can skip the queue for the check-in desks. You can join the queue for security straight away provided you have checked in online and do not have any baggage to drop off. Make sure that you have your boarding pass ready, either printed out or on your phone. You can see your gate number on your boarding pass, so you know which security check queue to join.