I am running NW 7.2. If I bring up the Networker Administrator under Windows and Edit the client and go to the Preferences tab (Yes, there is a preferences), there is a Server Network Interface. This must be set properly to use the network that you want to use. My Networker server is rs6kpsd5 (100Mb) and a Windows client is stpi06(100Mb). Both have a Gb interface. If I set the stpi06 client up for rs6kpsd5, then it will use the 100Mb interface. If I change the Server Network Interface field for stpi06 to rs6kpsd5s (1000Mb), then it will use the Gb interface.
Also, this particular client (stpi06) has about 3 million files in one volume (250 GB). It backs up over the Gb interface in about 8-12 hours. So, if you have 9 million files, you could expect ~24-36 hours, if you are getting rates like we are. This is during a production day, so there are users getting to the files while we are backing it up.