Do ethernet cable signals deteriorate the longer the cable is? I need to purchase an etheret cable to connect a second computer in my house, planning on running it through the attic. The wireless I am using is far too slow.
Yes. But in your case it won’t matter since the effective range of Ethernet is a several hundred feet. I doubt your attic is that long/big 
Tim
From memory the specs says the cable can be up to 100Meters.