The phone system used to have three extension sockets but as these are redundant with wireless handsets they have been disconnected.
Just to clarify, by that do you mean:
1. the phones have been unplugged from the extension sockets, but the extension sockets are still live (you get dialtone if you plug in a phone); or
2. the extension wiring has been completely isolated from the master socket
Option (2) will give you the best speed.
Option (1) *can* cause reduced speeds, depending on what state your extension wiring is in. It's always a good idea to isolate it for a while to help narrow down the problem.
In newer setups, removing the front faceplate from the mastser socket isolates the extension wiring (if it was installed correctly). In some older setups, the extension wiring was on an inline plug+socket that you just pull out. In very old setups, anything is possible