On arrival into MEL, everyone queues in the same line to approach the exit (this is generally were the delays are experienced). At the head of that queue is an officer that checks your card, may ask some quick questions, and directs you to a particular line. if nothing to declare there is a chance you may be directed straight through the exit doors. Otherwise you may be directed to an x-ray queue or if declaring something to the appropriate agents.
Generally its quicker if not declaring, but in my experience the biggest delays in MEL are the queue to get to the decision point.