I built a new PC for my across-the-road-neighbour and installed it yesterday. It came with built-in wifi, so I checked to see if we could improve broadband speeds by attaching the router (BT Home Hub 6, I think) to the main socket downstairs instead of the extension upstairs. I was unimpressed to find their ADSL connection running at 1.7Mbps upstairs according to speedtest.net, shocked to find it was 1.8Mbps with the router downstairs, disgusted to find that the speed check via their MyBT login confirmed 1.8Mbps and classed it as "good". I already knew the broadband checker tool gave the ADSL range as 5.5-12.5Mbps. When I was on ADSL back in 2012 I used to get about 17Mbps.
The main socket is something installed in the 1980's, and the extension was connected via a socket-doubler. The extension is now disconnected, and there's just the BT filter connected to the socket, with a cordless phone base unit and the router plugged into it.
What really puzzles me, though, is the readings I see in the DSL checker at https://www.dslchecker.bt.com for my neighbour's number. They're on ADSL, but the "observed speeds" section gives VDSL speeds as 1.99/0.79Mbps for today's date, and ADSL as 10.08/0.45 for July 2018, (which is possibly when they got the new home hub). Might that indicate there's something wrong with the way they are connected, rather than the physical connections?