If you have reverted to factory defaults and made the minimum configuration settings for it to work with your ADSL connection, and it STILL runs slowly - even with firewall and wifi access point off - then yes, it could be the router.
Could it be the cables
What cables are you referring to? RJ45 cables between router and PC? Cables between wall socket and router?
What cables are NOT used by the BT voyager that ARE used by the NetGear? I assume that the only difference would be if the Voyager is USB only.
In that case, it could be the network card on the PC. Either faulty or mis-configured. Check the system log to see if there's any errors / warnings associated with the card.
Try uninstalling the driver and reloading - ensure that you note all your network settings before doing so. See if you can find an updated driver for it.
Check the correct network settings for the NIC - if you are running the router at 100Mbps full duplex and the NIC at 10Mbps half duplex you won't get much throughput!
SD