From http://www.reddit.com/r/Bitcoin/comments/1r4d6t/bitstamps_streaming_api_and_exploitation/

...the stream I'm watching includes both order and trade events, and I typically see orders on the stream anywhere between ~2.5s and ~8s ("the window") before they match and produce a trade; if you're only trading by watching BitcoinWisdom and others, I see things happen as much as 18s ahead of you.

So if you are not trading looking at the undocumented API you are beign ripped off by those who do. Very nice feauture!