To start off, I am a professional network tech: A+, N+, S+, CCNA, CISSP, CASP, among others. I know how to diagnose networks for a living, which is why I am baffled.
I have a 50mb connection, and an enterprise grade 3x3 MIMO b/g/a/n router. I can play games with no lag, I can stream Netflix/Hulu/Amazon in full HD with no buffering problems whatsoever on my 360, I can stream media from my NAS device on the LAN at 100mb wired or full 802.11n speeds. On my laptops, computers, and other devices I always hit my 50mb connection speed; it's entirely reliable. The only time the connection is ever noticeably slow is when I'm buying games/demos/DLC in the XBL marketplace.
It's excruciatingly slow, we're talking around an hour a gig. I've properly configured all the port forwarding, static IP, gone through every diagnostic I could dredge up just to see if any of it did anything or could identify a problem; it did not. I even tried plugging the connection straight from the cable modem to the Xbox with a Cat 6a: same problem. Downloads and ONLY downloads are slow as ****. At this point I am utterly stumped. The only thing I can imagine is that Time Warner Cable is packet sniffing and shaping the traffic, but even SSH tunneling through a fiber-connected VPN did nothing. Do I just have a bum Xbox? Is there anything you guys can think of?