Alright, so I am trying to create a picture in my head of how your network is currently setup.
First you say that you have one computer which is connected to your hub. (understood)
Then you have your second Xbox when it is connect, connecting via wireless.
QUOTE
for reference the other xbox connects wirelessly to a wireless access point which is connected to the same hub
I believe you problem may lie with your network setup. If i understand correctly you have your modem which is then connected to your Hub which then feeds your router?
If this is the case, try changing it around and connecting the modem to the router and the hub into the router instead. The router will give you the full bandwidth of your line and not limit you to low speeds that the hub will.
As for why this may be the problem I'm not sure exactly as i do not know your current setup but it would depend on if your computer is connect into the router (wired or wireless) or connected into the hub. I'm assuming you probably have it connected into the router, which is good but then you have your xbox connect to your hub which is probably your conflict.
I'm not 100% sure on the above, as i do not know you current situation perfectly.
But i would give it a try and let me know how everything works out, i will continue to give you idea's if needed.