Interesting to hear that your speed has dropped. I have a customer on a long line and they have had major problems but only since the exchange was upgraded from 8Mb to 20Mb. I think the infrastructure was able to cope better with slower line speeds.
We even tried LLU broadband from a different ISP and although that worked for a couple of months the speed went from 750Mb to virtually non-existent. They said that there would be a maximum of 20 people using the LLU line which seems remarkably little but obviously even that was too much with a long line!