Retreating blade stall
Nickel
Phill77's reference to Wiki gives a very precise account.
Low rpm only comes from not enough engine power.
Retreating blade stall comes from both lots of rotor power (and so engine power) and lots of speed.
So the only correlation between low RPM and retreating blade stall is that both are influenced by the power factor.
Constructors VNE kind of takes worst case of all parameters (density, TOW, speed)
The R44 states 130 VNE, but in many practical flight conditions retreating blade stall will not be significant until 150-160 knts. In all my experience MR rpm is the most significant factor : a few % can make a big difference (for instance using 95% instead of 102% will create retreating blade stall in many normal R44 flight conditions)
Retreating blade stall is by the way in fact "retreating blade tip stall", because close to the hub the blade always stalls at some point in forward flight.
d3