If this is strictly internet related, I couldn't really say what the issue may be. However, if you have similar network performance discrepancies between the two OSes, you may want to look at OS X's Network Utility. Under the appropriate selection (probably "Network Interface en0") in the Info tab, you should be presented with all pertinent information, such as MAC address, IP address, link speed, etc. Also, on the right side of that is Sent, Received, and Collisions. If you're getting lots of collisions or a low Link Speed (10Mb on a 100Mb connection, for example), you might want to look at changing your cable or try maybe investigate your other network devices (switches/hubs/routers/etc).