To clarify how your line is made up - you have a dedicated pair of wires from your house to the exchange, possibly several kilometres worth.
Along the way back to the exchange, more and more pairs of wires will be in the same cable, typically up to 100 pairs of wires as it enteres the green cabinet at the side of the road, then in 200 or 400 pair cables back to exchange. But you still have your own exclusive pair of wires all the way back to the exchange.
This pair determines your sync rate and line stability.
At the exchange, your line splits, via microfilters (in effect), voice goes to the exchange linecards, data to the DSLAM. The DSLAM has a number of Backhauls going off via the BT ATM network back to POPs, then from these to your ISP via Centrals (normally 622Mbps), then through the ISP's network onto the internet via their transit links.
If you get a good, fast, stable sync, but poor throughput in general, this is generally down to either exchange congestion on the backhauls (which would affect multiple (but not necessarily all) ISPs), the ISPs Centrals (most likely), or the ISP's own network and onward transit links.
Very simplistic view, but should be enough info