I normally get around 7.5Mb/s from BT, there was a period when it started going down to 1.5 at night, rang the help line and an engineer appeared next day checked everything through but found nothing wrong, xo he installed a broadband skt in place of the old one but said it;s unlikely to make any difference but I have to do it. The problem remained so I rang again and the same Eng arrived and said try that, I've put you on a different rack at the exchange, been perfect ever since.
I know Mixture hates them but I've never had any serious problems with them.