Exactly the same issue. Moved to Smart Hub 2 (with discs) in October without a problem. Connections started slowing and intermittently "pausing" the connection, both wireless and wired via Powerline adapter.
Ran SpeedTest from the BT site and it reported 32/18Mb (was 72/18Mb with the old router). Plugged directly into the Gigabit port on the back of the router and repeated the test. Same result. Ran using Speedtest (which shows progress) and it starts around 65Mb and then degrades over 3-5 seconds to 25-30Mb. Got a replacement hub and exactly the same problem. Plugged the old hub back in and "bingo", 72/18Mb.
BT say that it can take up to 3 days to "train" the router. I may just put the old one back until after Christmas and then run more tests.
I suspect a firmware glitch (it was apparently v0.17 in September and is now v0.24).
Watch this space.