A modern LCD TV is built with many components.
And it behaves like a computer:
1) When turned on, a bootloader inside the flash is started.
2) The bootloader checks if all components are OK.
3) The bootloader then needs to move the remaining firmware from flash to memory.
4) Then the firmware starts, which is actually almost an OS - on Smart TV's.
5) The firmware needs to start tuner, MPEG2/4 engine, video processor, audio processor, surround processor, etc.
6) Meanwhile it starts showing the picture of the last set input.
7) The video/audio stream needs to be buffered and gets sent to the video/audio processors for all sorts of enhancements
Finally the OS has finished booting and loading all the SmartTV stuff.
9) The front processor is now ready to receive commands from the remote.
10) If the DVB-S/S2/C/T channel is witched, the MPEG decoder needs to wait for a key-frame, which can take up to one second.
This is just a rough list of all that is going on inside a modern TV!
Better TV's may have some advanced logic to allow fast switching on channels of the same transponder, by keeping a simultanous tack of each stream.
But if you change transponder, you will have to wait again for the tuner to lock in and the key-frame to appear.
Regards,
Vitor