Distance is everything with VDSL, an extra 20 m of telephone wiring can mean a couple of Meg lost.
.. or more depending on your connection. I gained about 5Mbps just by replacing the standard flat cable that came with the modem with screened Cat5. 
This is often due to a
reflection loss rather than a
wire loss.
As a percentage of total loop length, the flat cable between the modem and the linebox is so short that signal attenuation due to losses in the wire itself is minimal.
However, the flat cable is poorly matched in terms of its
impedance to the twisted pairs in the rest of the loop. That impedance mismatch causes much of the signal to reflect back to the modem transceiver/s from the junction of the two cable types. And that reflection is what impairs the broadband performance. It causes frequency-specific notches that ultimately reduce bit depth on the affected subcarriers.
That is reportedly how most bandwidth is lost to consumer wiring: as reflection losses. Compounded by star wiring topologies, which serve as reflective bridge taps, etc.. (See
Walter Y Chen,
Home Networking Basis)
cheers, a
Edited by deleted (Wed 27-Feb-13 00:51:30)