First, you need to check the router model. If it's 802.11B it's old and slow. G is faster and N is even better. BUT your network will drop to the slowest device on it. Also, signal strength drops greatly the further you get from the router. Try to get the router up high (like on top of a cabinet) to help.
Second, DSL speeds are dependent on distance from the phone company hub. In a given neighborhood (village) the residence closer to that hub will get better signal than one at the far end. (Understand this is simplistic and there is much more technical stuff involved)
Centurylink claims "up to 40Mbps". Comcast claims "up to 50Mbps. I can guarantee that you won't come close to that. Google "bandwidth tests" to get a number of free test sites. (speedtest.net
, cnet.com/internet-speed-test
, etc). Comcast gets a lot of bad rap here, but cable will probably give the highest bandwidth and speed. DSL and cable both can give high rates, but both (providers) can and will cap the bandwidth in order to accommodate more customers and higher use at certain times. As ZCaveman said,You will get what you pay for.
I have Comcast and Directv, and usually get download near 30Mbps, but tonight I'm only getting 15.