When the local Teleco installed (or upgraded) interwebz connections they used to automatically replace the old telephone cabling in the house with CAT 5 network cable as it gave better perfcormance.
When I bought the new apartment and had Interwebz installed I was disappointed with the speed and the teleco doesn't upgrade the cable any longer.
I discovered that I couldn't upgrade the cable myself as there is a blockage in the conduit between the telephone point that the modem is connected to and the junction box outside.
Yesterday I had the brilliant idea of cutting the connection between the phone point with the modem connected and the other points in the apartment (we don't use them).
Hey Presto! Upload and download speeds doubled!