To determine if it's your local network (LAN) or your ADSL supply network, can you hook directly into the ADSL bypassing any routers, especially wireless routers?

If you can connect to the ADSL, run speedtest.net and look for a few more network testers and compare them to see what rate you get. Most speed testers will not show you're getting the advertised rate. Contact your supplier and ask what speeds you should get.

If a direct connection looks good, the rest is within your LAN, which if you're using a router should be much higher speed/rate than the external connection. Time the transfer of a file of known size from one machine to another to get a rough idea how fast it is. A video file is a good choice... something a gigabyte (or 500meg) big may be easier to time than small files as big files give you a better chance of starting and stopping a timer. (Compare timing the fall of a coin 10 centimeters with dropping the coin 10 meters.)