I got BT Infinity a year ago. When first installed I was getting a speed of 25 that it now down to 17. You don't need tell me about BT Customer services via the telephone. Worse than useless.
BT have not been able to give me a reason for the drop in speed. From research it appears to be because more and more people in the near vicinity have switched to BT. Ergo more users slower speed.
Will be interested to see responses from others.