In many smart TVs, when you press the input button, it doesn't immediately pop up a mini OSD. It loads a program that can take as long as it does to load a streaming service's app. This program shows you ads, sometimes even inserts ads for streaming services in between the input sources you have to click over. This is before the time it actually takes for the input to change. With my TV, if I am on TV/Antenna, it takes me about 10 seconds from the time I first press the input button to when I can press the OK button on HDMI 4 which sends the command to switch inputs. The actual switching of the input takes about 1 second.
But not all. So, clearly it doesn't have to be below the OS level. I agree it needs to be quick, responsive and reliable, but that doesn't preclude it from being implemented in the OS.
To be fair I used the word "should", as in I believe that it would be best practice, at least from the perspective of user experience, to operate below the OS. If that were the case, input switching wouldn't be dependent on the speed or reliability of the OS. As you point out, it's not strictly necessary if the OS is responsive enough and reliable enough to begin with.
The fact that I have to wait for my TV to boot so that the 'input' button works in order to switch to the device I intended to use when I turned it on.
Mine takes 2-3s to come up, and at that point switching inputs takes pretty much only the time it takes for me to choose it. So, again. The problem is not that it has to be below the OS. The problem are crappy OSs and/or hardware.
Input switching on my TV doesn't even cross the Doherty threshold, or at least not in any way that I notice. And believe me, I get as irritated as anyone with laggy TV menus - which is one reason I refuse to buy Samsung at this point.
The speed and reliability of the OS. I should be able to change inputs even if the TV OS is unresponsive, which is not uncommon on some smart TVs. If the smart TVs you're using are fast enough and reliable enough that you aren't having issues then I don't think there would be any benefit to you.
As opposed to what?
On my LG there's a button for input switching that does only that. It's part of the OS but I don't see how it could be more convenient.