It depends more in the way you receive your signal and if you have some kind of set top box in use. The TV is the last thing that has something o do with the channel changing time.

For example if you have IPTV (ATT Universe for example) it can take longer to switch the channel than for example cable due to the IP Multicast technology used for IPTV. In the old TV world every channel is broadcasted simultaneously to you, meaning the signal of every channel is always available. With IPTV where each channel needs about 2 - 4 Mbit/s (SD- HD) you can not transmit every channel simultaneously. So every time you switch the channel the set top box sends a signal to the IPTV head end to request the new channel. This takes a a while (request has to travel to the head end, head end switches channel for you, signal travels to you). A way to speed up the process is to transmit all channels for example to the DSLAM and the DSLAM does the channel switching.