As others have said, DSL is sloooow.
Plugging in a computer via ethernet will help that one computer and possibly help your other wireless devices due to less wireless congestion but, you are ultimately limited by your DSL speeds. Your plugged in computer (in fact, any device connected to the network) can easily use all of the available bandwidth of standard DSL connections (typically 5-10Mbit) just by streaming videos, playing online games, or downloading content.