I don't think that RadarVirtuel has much of a delay, if any. If I hear a particularly noisy takeoff when I'm sat here in Stortford, it almost immediately shows a 747 heading off.
But it is no use for old aircraft or bizjets, or, I suspect, Russians.
By the way, I got so bored with IE 7 problems with RadarVirtuel that I decided to give Chrome a trial. I have not switched back...