I've just moved into a new house (not a new build I hasten to add) and used the existing phone line and BT Master socket, initially I saw speeds of 43Mb but as the line stablised this dropped to mid 20s and then to mid 10s, at which point (a fortnight ago) I called BT and an engineer was sent out. He found no fault with the house to the street and then traced it from the exchange to street where a poor connection in the junction box was diagnosed. The line was reset and I was back to the 40s again, which then started to drop.
I've reported this on two occasions and had the fault closed by BT, I raised a complaint and now have an engineer coming out later this week, I'm puzzled as to why the fault has been closed and required time and effort on my part to escalate the issue to get an engineer to attend.
For those interested:
TP-Link Archer VR900, configured and working as it was at previous property.
All extensions were removed by previous engineer to eliminate line noise.
I'm sorry for the problems you've been having with your broadband recently. Can you post back and let us know how things go with the engineer? If you're still having problems after the visit we can pick it up and have a look at it for you from here.