I have a 900 Central, I play XBOX Live, play BF3 and a lot of shooters online, I have not noticed any lag at all.
Now, I have the ports of xbox live forwarded in my router. After reading this post, I did a few tests:
1- BF3 ( this is the game that I play most) and PC streaming some videos, Netflix.
No lag in BF3, played smoothly and no issues with Netflix, tried both, wired connection (from one of the ports in the back of the 900C) and wireless, with the wireless, I noticed that It took a bit longer to connect to the servers, but once the game started, no lag.
2-GR, soldier of future and downloading some files from the net (…Some files…Torrents) a bit of lag, nothing big, I was able to adapt and the lag did not make the game unplayable.
3-BF3 and downloading some files (again, torrents) lag, but I was expecting this, as the torrents were taking a lot of bandwidth, I limited a bit the download/upload rate in the program and I was able to play BF3 at a tolerable level ( got killed with shotguns in the **bleep** scrapple metal map…HATE that map)
4-NFS on the run ( I know I know, it was a gift…) , here, the game was lag free, I was streaming in the computer and then tried with downloading few files, still the game ran lag free.
5-Saints Row the third and netflix, here, I have lag, to the point that kicked my co-op friend off the game.
Once again, I have the ports of the Xbox Live service forwarded in the modem, I don’t know if this make any difference. Also, I have the router in AP (access point)