The same process, but in a different scenario, was observed in supernova 1987a: "Approximately two to three hours before the visible light from SN 1987A reache

The same process, but in a different scenario, was observed in supernova 1987a: "Approximately two to three hours before the visible light from SN 1987A reached Earth, a burst of neutrinos was observed at three separate neutrino observatories. This is likely due to neutrino emission, which occurs simultaneously with core collapse, but preceding the emission of visible light. Transmission of visible light is a slower process that occurs only after the shock wave reaches the stellar surface." This result set a pretty strong upper bound on the neutrino mass because it must be small enough that, across the 168,000 light year distance from the LMC to us, the neutrinos kept their lead on the photons emitted a few hours later. A neutron star is much smaller, the process is much faster, and the event happened much further away, so the constraints on the difference in speed between the gravitational waves and photons should be even more tightly constrained in this event.