[MovedGoalPosts]

Most users will see a performance increase by swapping from USB to ethernet, simply because ethernet uses dedicated hardware on your PC rather than general PC resources under USB.

I'm not quite clear from your post exactly how your are connecting, but it sounds like you are not using a router, and thus you are trying to use internet connection sharing. For ICS to be successful your master PC needs two ethernet based network cards.

Using ICS your wiring via ethernet should be:
modem <> master PC network card #1
master PC network card #2 <> second PC network card.

You will need to establish the internet connection on the master PC (remember to reboot that modem and the PC). Once you have the master PC setup OK, you then need to run the Internet Connection Sharing Wizard (check under network connections in Windows).

NB thread moved to Networking discussion