You have a Microfilter - Do you have a spare you can swap it out with?
Also consider testing plugged directly into the router. Google Speedtest.
Also remember that speeds drop at Peak Times like now - This is because more people use the Network.
If you cant figure it out... Speak to Plusnet and ask them to run some line tests. PSTN and TAM Testing. That way the can detect for a variety of things.
HR Joints, Dis in Networks, Battery Con Faults, Crossed Lines, Voltage Impurities...
Dont worry - you dont need to understand all of the above... Plusnet should do though... And site of the above means BT.
But be warned, if BT come out and find its your kit then you risk £160 + VAT as a charge if its an SFI Engineer (Broadband - Youre reporting slow speeds)
Examples of Sockets and Filters below;
This is a filtered faceplate... No need for a Microfilter Plusnet router will connect to the left socket... The rest you get the idea...
Filter with faceplate is below
Don't touch them owe me £500 since January 2019 make excuse after excuse. Seem they always have software problems sending money out. Keep saying they will call back or email nothing been chasing it now for 6 mths the phone staff always have the same banter we will chase it up and get back to you then nothing!