According to Quantum FFF Theory, Black Holes have no inertial mass, due to their non Baryonic Dark Matter Axion/Higgs content, (then no rotational strain but only nuclear particle oscillation-tension effects will take place during merging.
Black Hole merging without rotation oscillation, due to massless BH Inertia absence, according to Quantum FFF Theory. (=duration about 1 sec.)
Neutron stars however have normal baryonic mass and consequently rotational based strain effects. (=duration about 30 sec )
The difference in duration (30 sec.to 1 sec.) is supposed to be originated by the fast transformation of massive Neutrons into massless Axion Higgs nuclear particles of the newly formed black hole during the merging process..