Okay the 50dB attenuation should give better speeds.
Looks like noise is an issue, now that could be a bad joint, water ingress, animals living in the cabinet
Or the more normal interference from something local.
Testing from the test socket removes the influence of extensions in the home, which can pick up noise, so worth a check.
Also I would plot the data on a map for all the people if you can, if it clusters around one area, then could be something as simple as a noise source from a welding business etc that keeps your speeds down.