It is a small amount and even when ib use you are unlikely to notice it. I have had mine router enable for years and have never noticed any problems although I doubt if it can be seen unless they are either in my garden or a neighbours house. I occasionally use it with no adverse effect to the main connection.
Do not worry about security - it uses a separate SSID and a VPN and users cannot see any of your network.
As mentioned, it will give you access to thousands of BT hotspots throughout the country which I certainly find useful when in moving around towns.
M H C
taurus excreta cerebrum vincit