By way of explanation of my previous observation that the A11 dualling scheme is flawed, the problem lies at the Barton Mills roundabout. But only southbound.
The two principal roads leading out of Norfolk towards London converge here. Given that we give way to the right, traffic on the A11 - however adequately catered for on the new dual carriageway - will always have to slow, if not stop, at this point. Much as it did previously at Elveden. It seems fairly clear that the new scheme will simply have moved the previous pinch-point closer to London.
A southbound flyover, for which there would appear to have been room, would have resolved this