agree with above about number of devices, most routers are well above actual connection speed so can keep up with what actually receiving. i beleive most lines still owned by BT and the exchanges? so unless being throttled by isp wont make any difference.
if your router is plugged into a socket other then master make sure extension socket is only connected with the 2 required cables within wiring. older installations had 3rd wire for bell on old phones still. can remember colours of required or numbers on terminals required. if 3rd connected it can pick up interference within house. i got 2 extra mb and cable was new that i put in, didnt realise when first connected that only needs 2 of the six now.
my router is on extension and gets same speed as master socket!