The article says it doesn't add any lag, but I am skeptical. At the very least there will be a buffer to sync playback with the display. Apple didn't design this feature with this use case in mind, so I doubt they put in any special effort to minimize latency.

In my experience, when random people say they "don't feel any lag" with something but don't put up any numbers to back it up, I usually end up feeling quite a bit of lag that they just don't notice for one reason or another.