If your question is for the purpose of ATPL theory study then you need to consider the gradient of the drag curve.
If the drag curve gradient is very steep then a small change in speed will result in a large change in drag. At speeds below Vmd this would make the aircraft more speed unstable. But at speeds above Vmd it would make the aircraft more speed stable.
Swept wing aircraft tend to low coefficients of drag, which gives them shallower drag curves compared to straight winged aircraft. This reduces the speed instability below Vmd. But it makes it harder to maintain a given speed at or above Vmd and this might be interpreted as an increased speed instability.