you have an internal wiring problem. as you can see the improvement with the change to the test socket which eliminates your internal wiring
have you removed the bellwire from terminal 3 in all your sockets?
using the picture below as reference, when I removed the front plate, there were no wires attached.
Some of the wires were pushed down inside the recess but two were sticking out.
Is that the bell wire?
from the pic it looks like the main wires 2&5 and the orange bellwire attached to terminal 3 the bellwire should be removed and also in the extension socket that is conected to
I took another reading. It's still much better (nearly double the speed) than I had before. This is using the test socket.
These are the speeds I usede to have before May 1st.
are you using the same filter? if you have no other extension sockets hence no wiring on back of faceplate.
it is possible that there is a problem with the faceplate. they don't go faulty often but they do go fauilty
I would leave in test socket as you have no extension sockets leaving there just now while connection settles with new speed will not cause you a problem