In your videos you are playing back different things from your virtual channels than you are from the library.
Please attempt to play back the same videos that are giving you trouble via virtual channels, from your library, as that is a 1:1 test.
The goal is to test if the exact same videos are loading the exact same way.
I can say though, that the video where you start the virtual channel from the guide, looks like the TV is mode switching, and that's why it's "lagging". The screen blanks out before coming back after the video has started, which is normal behavior for Content Matching on the Apple TV.
The movie you played from the library had no mode switching, so it played immediatly.
So maybe what you're experiencing is the mode switching on your TV from Content Matching.