Everytime you tune/change a channel emby runs ffprobe for a few sec to figure out which video and audio stream are available.
I could be wrong but at least for hdhomerun atsc tuner. I think this information remain static. Which means that the scan process (or a daily scan) might be able to figure out the available stream once. And then rely on that stored information next time it starts to play.
In my mind that would make live tv without transcoding super fast.
Emby Theater desktop does not probe anymore.