If I connect an Ethernet cable, will my Mac automatically select the fastest route for local network traffic?
You want to make sure that your Mac's ethernet device is before the wifi device in the system's network service order. In System Preferences, Go to Network; under the device listing, click the 'gear' icon and select 'Set Service Order.' Drag 'Ethernet' to the top. Per my understanding, all traffic will attempt to be routed through the top-most device, and will only be routed through devices lower in the chain should the above route(s) be unavailable.