I have a Dell 2155CDN Multifunction printer that is hooked up through our network. But when using the scan feature, it is terribly slow, and I assume it has something to do with it scanning through the network port. Is it possible to leave the printer hooked up across the network, but also attach it to a computer via USB just for the scanner? Or would doing this confuse the printer not knowing which port to use for each feature?
The printer will never confuse the USB and network ports. Ports are served on a first-come-first-served basis. The printer will start on whichever port becomes active first, and tell any other port(s) to wait.
I have found a similar lack of speed with my Xerox CM305DF. Network scanning (and printing) is unacceptably slow, so I use the USB instead. While printer and PC run 1000Base-TX, my router is limited to 100-Base, so that may be the problem. I reported this to Xerox but they could not come up with an answer beyond "usb is faster than network".
As @ultrasawblade points out, that means I have 2 drivers, but I have set the USB driver as my default for both printing and scanning. If there is still confusion you can easily rename the network driver to something like "do not use".
Probably the worst that can happen is that you find you have two printers in your Devices and Printers (assuming Windows 7). One will be what it found via USB and the other will be what it found over the network. Your scanner software may also look it it found two scanners.
I don't know if the printer itself will "shut down" the Ethernet or Wifi interface if it detects a USB connection. This is the "cheap" way out from a programming standpoint and I'd place my bet on this happening if both are connected. This is also a way more likely possibility than the printer getting "confused" by receiving two documents from two different interfaces at once.