This, which can be found referring to any speed limit, not just 40mph, was actually said by a councillor recently in an attempt to oppose a cycle lane on a wide main road.
As far as I know, there is no law of physics that determines that a road must have any particular speed limit (and I do have a PhD in physics). In fact, a simple calculation shows that reducing speed from 40mph to 30mph costs a maximum of 30 seconds per mile. On a 5 mile long road, that would be 2.5 minutes, even if it were possible to travel at the speed limit for the whole length, which it usually isn’t.
Compromising the safety of people on cycles and on foot – of children and elderly people – for the sake of delays that are in practice considerably less than 2.5 minutes seems like the action of a pyschopath, and at the very least is incredibly selfish.