We're excited to announce that we've fully migrated the Xbox Forums to their new home, here on the Microsoft Community forums!
As of today, going to forums.xbox.com will redirect you here to the Xbox area of the Community forums. We've moved all of your existing threads over so you can continue and start conversations with the new forums experience.
We sincerely appreciate the feedback you have provided over this transition and can't wait for you to check out the new Xbox Forums!
You're not actually getting 100Mb/s or 54Mb/s on your connection. Those speeds are the maximum speeds of your networking equipment, not the actual speed you're getting from your provider. Perhaps, you could do this
speed test and post back your results?
That would explain your buffering problem. 0.55Mb/s down is below the minimum requirements for Xbox LIVE. The minimum is 2Mb/s down, and even at that speed you would still have buffering issues watching HD videos.
You'll either need to contact your provider or change providers. Those speeds are not quite good enough for Xbox LIVE. Sorry.