The flight info comes from a network of private ADSB data receivers. There is a delay on the web position info from actual. I built some home brew antennas and with a USB SDR reciever, track them locally with real-time position.
I watched three separate flights yesterday on the track map and went outside to check the sky for each one when I expected them to be overhead and there they were; right position and right heading. There was very little latency that I could tell.
I did get kicked after a while though. It wanted me to sign up, and probably wanted money. :-/