Well, encryption is certainly part of the story. DVB encryption uses messages called ECMs to distribute the key that is used from the receipt of the ECM through to the receipt of the next ECM. The time between two ECMs is called the crypto period and is generally in the order of a few seconds. The average wait time to get an ECM on changing channels is of course half a crypto period.
The other major source of delay is video compression, as defined by MPEG. MPEG uses bi-directional compression in the time domain, which means that frames are sent out of order, which in turn means that they have to be buffered. Furthermore, a complete frame, known as an I frame, is only sent every 10 or so frames (it depends on the profile of the transmission channel - is it lossy or not for example). As the P and B frames are basically diffs from the base I frame, you can't decode an image until you have received an I frame. This too introduces a delay of typically 0.25 seconds, and is the reason why even unencrypted digital is slower for changing channels than analog.
The other major source of delay is video compression, as defined by MPEG. MPEG uses bi-directional compression in the time domain, which means that frames are sent out of order, which in turn means that they have to be buffered. Furthermore, a complete frame, known as an I frame, is only sent every 10 or so frames (it depends on the profile of the transmission channel - is it lossy or not for example). As the P and B frames are basically diffs from the base I frame, you can't decode an image until you have received an I frame. This too introduces a delay of typically 0.25 seconds, and is the reason why even unencrypted digital is slower for changing channels than analog.