More than likely you have a system hold time of 2 seconds (so it spends 2 seconds scanning and rescanning each channel in each system before moving on to the next system).
If you want to test how long it actually takes to scan the channels, put the local weather channel into the system. With it stopped on a weather broadcast, press SCAN and time how long it takes to return to the weather broadcast (assuming no other channel is active, this will do a single scan of all channels in that system, from which you could derive the scan speed).