Intervention time for low-g is going to be important, but so long as you re-load the rotor as soon as you start to feel light in the seat, you are probably doing as much as you can, and although it is generally too dangerous to actually train for, it is rammed home as a killer under the topic of 'mast bumping' and even the slightest feeling of low G is enough to start alarm bells ringing in a teetering-rotor-helicopter-pilot's head. I have no direct knowledge but would guess the intervention time is not very different in different types.
However low rotor RPM in low-inertia rotors (like the R22) does have very short intervention time, as little as 2 seconds in certain flight regimes, but the only required action is to push the collective down, rolling on throttle and maybe a flare. The R44 has a much longer intervention time (10 secs? (guess)) and the Jet-Rangers/Hueys longer still. It is mainly a function of rotor mass - the heavier the blades the higher their momentum and the longer they take to slow down. R22s have light blades, and a very loud horn goes off if rotor RPM gets down to 97% of normal.
If the blade pitch is not reduced to reduce drag and allow the engine torque to increase rotor RPM (or use the upward airflow to start autorotation if the drive has failed) then once the rotor RPM reduces below about 75-80% of normal RPM, the upward airflow as the helicopter starts to fall increases the aerodynamic angle of attack on the blades. They can stall and the increased drag acts as a massive rotor brake which exceeds the engine torque, so you can't speed them up even on full throttle. In a low-RPM accident, it is not unknown for the blades to have stopped, or almost stopped, rotating by the time the aircraft reaches the ground. All rotary-wing-pilots know this stuff.
None of the above is speculation on this incident, which (as is often the case) may turn out to be another series of unfortunate minor causes with a very sad outcome.
BW