Re: Infinity 2 slowing down and not near the initial estimate.
When you signed up the estimate you were given would appear to be in the Range A. This seems to be common practice with BT sales only quoting the Range A speeds. I don't know if this is by design or lack of information on behalf of the sales people. They should quote you the speed from the A and B range which reflects the best and worst you will get.
It looks as though you are in the lower side of the Range B.
When you first get Infinity installed it runs at the maximum the line can handle. After a day or so DLM (Dynamic Line Management) then starts to adjust the speeds to what it deems is the best speed to maintain a stable connection. This can often mean that that the speed is lowered, as would appear to be what has happened to you.
It could be noise on the line or a bad connection that is keeping the speeds low. You should complain to BT and request that an engineer investigates the problem.You might have a problem getting this because BT will state that you are within the estimated range. You need to be persistent with them about getting an engineer out and don't take no for an answer.