Is there anything else on the hub with the XBox and PC? Are you doing anything with either at the same time?
A hub has "shared bandwidth". Meaning, if you've got a 10MBit hub, with 5 ports, and someone is using up 7MBit, you've only got 3Mbit to share between the other 4 ports.
So if someone is downloading some huge files and hogging a lot of the bandwith, it could cause a slowdown on your end. I'd suggest that you test by connecting your XBox to a Crossover cable directly to your PC. This will elimintate your network hardware, or any bandwidth problems. If your getting the same transfer rates with the crossover cable, then I'd move on to troubleshooting your NIC.
Of course, I'm assuming you've tried to find the latest drivers. That fixes lots of problems.