Nr decay
If your nr decays then it means your aoa is too much therefore the drag created will slow the rotor continuously until either you lower the lever or you crash.
If the aoa is too high then you run the risk of stalling the blades/aerofoil. Once you stall it just goes from bad to worse and recovering the nr will become impossible beyond a certain point. I guess this is the point you are referring to.
Forward speed is irrelevant, once you stall the rotor it is quite often game over.